Add a storage for facetSource specific config

1. +++ b/src/Controller/FacetSourceConfigController.php
   @@ -0,0 +1,42 @@
   +   *   A renderable array containing the form
   

   Missing a dot?

2. +++ b/src/Controller/FacetSourceConfigController.php
   @@ -0,0 +1,42 @@
   +      // Return a renderable array with a short lifetime that represents the plugin not found.
   

   Comment longer than 80 chars

3. +++ b/src/Controller/FacetSourceConfigController.php
   @@ -0,0 +1,42 @@
   +      return ['#markup' => 'Plugin not found.', '#cache' => ['max-age' => 5]];
   

   If we do not provide the cache param, does that mean it might be cached indefinitely? Should we add that here so that people understand why?

4. +++ b/src/Form/FacetSourceConfigForm.php
   @@ -0,0 +1,79 @@
   +    $overridden_filter_key_config = $config->get('overrides.' . $facet_source_id . '.filter_key');
   

   Is there a way how we can make this "pattern" more global so that we don't have to have internal knowledge where this is referenced but rather get a certain class variable / constant that defines what the pattern is? It's "antfucking" but I don't like it that we do this concatenation in multiple places.

5. +++ b/src/Form/FacetSourceConfigForm.php
   @@ -0,0 +1,79 @@
   +      '#markup' => $this->t('Saving this form will create a configuration override for this specific facet source. Not doing so will make sure that Facet API uses the default settings. This is an advanced feature and unless you are fully aware of why you\'re creating this configuration. With a default configuration of Facet API you can leave the default configuration.')
   

   Isn't there a convention that we can wrap this text in multiple lines? It mentions "Unless you are fully aware of why etc... but it doesn't really mention what happens unless. Please rewrite to be more clear.

6. +++ b/src/Form/FacetSourceConfigForm.php
   @@ -0,0 +1,79 @@
   +      '#default_value' => (!is_null($overridden_filter_key_config) ? $overridden_filter_key_config : $config->get('filter_key')),
   

   Should we check if it is_null? what if it is empty but not null?

7. +++ b/src/Form/FacetSourceConfigForm.php
   @@ -0,0 +1,79 @@
   +    $configKey = 'overrides.' . $form_state->getValue('facet_source_id') . '.';
   

   Can this form value ever be invalid as a configkey string? Any sanitization we need to add?

8. +++ b/src/Plugin/facetapi/processor/ExcludeSpecifiedItemsProcessor.php
   @@ -56,18 +56,18 @@ class ExcludeSpecifiedItemsProcessor extends ProcessorPluginBase implements Buil
   +    $config = $processors[$this->getPluginId()];
   

   How is this related to this patch?

9. +++ b/src/Processor/UrlProcessorPluginBase.php
   @@ -41,10 +42,26 @@ abstract class UrlProcessorPluginBase extends ProcessorPluginBase implements Url
   +
   

   I'd remove this new line

10. +++ b/src/Processor/UrlProcessorPluginBase.php
    @@ -41,10 +42,26 @@ abstract class UrlProcessorPluginBase extends ProcessorPluginBase implements Url
    +    $override = $facetSourceConfig->get('overrides.' . $facetSourceId . '.filter_key');
    

    Another "dark magic concatenation"

1. +++ b/src/Form/FacetSourceConfigForm.php
   @@ -0,0 +1,79 @@
   +    $form['description'] = [
   +      '#markup' => $this->t('Saving this form will create a configuration override for this specific facet source.
   +      Unless you have to override facet source specific configuration, you can leave this to the default settings.')
   +    ];
   

   Maybe this documentation is too technical and not really needed for sitebuilders, we could add it in the code though?
   In here we can maybe add something more basic like : "Global settings for this facet source and all facets belonging to it." or just leave it out.

2. +++ b/src/Form/FacetSourceConfigForm.php
   @@ -0,0 +1,79 @@
   +      '#description' => $this->t('They key used for filtering in the URL, defaults to f.
   +      You should change this to something else if you expect to have multiple facet sources on one page.'),
   

   Maybe as description: "The key used in the url to identify the facet source. When using multiple facet sources you should make sure each facet source has a different filter key."

One global remark.
Using the overrides system creates only 1 file for facetsource configuration when exporting config.
This *might* make this configuration more conflict-prone. Not sure if this issue is big enough to change the design though, facetsource overrides shouldnt happen often?
It would be nice if every override lived in their own configuration file though ;)

Let's discuss this tonight.

IMO we can not allow stale configuration at all, so we will need to find out a clean way on how to delete overridden config for non-existant facet sources.

1. +++ b/src/Entity/Facet.php
   @@ -371,6 +379,40 @@ class Facet extends ConfigEntityBase implements FacetInterface {
   +    // plugin, so we create on on the fly; trough magic.
   

   typo. Should be through. Also it mentions twice "on", probably should be create it on the fly

2. +++ b/src/Entity/Facet.php
   @@ -371,6 +379,40 @@ class Facet extends ConfigEntityBase implements FacetInterface {
   +      [
   

   does Drupal code standards allow short-hand arrays here? I am still confused what the recommendation is.

3. +++ b/src/Form/FacetSourceEditForm.php
   @@ -0,0 +1,90 @@
   +      // plugin, so we create on on the fly; trough magic.
   

   same typo

4. +++ b/src/Form/FacetSourceEditForm.php
   @@ -0,0 +1,90 @@
   +    // Set the module handler. This is usually handled for use, but because of
   

   More English typos. "This is handled for us". Also I do think this needs more clarifications what "usually" means and where an example can be seen.