Add an availability to add/edit tampers to sources

Good work. I was able to add and edit Tamper plugins for a feed type (as superuser).

A partial review:

1. +++ b/feeds_tamper.routing.yml
   @@ -0,0 +1,25 @@
   +    _permission: 'administer feeds_tamper'
   +    _custom_access: '\Drupal\feeds_tamper\Form\TamperAddForm::checkAccess'
   ...
   +    _permission: 'administer feeds_tamper'
   +    _custom_access: '\Drupal\feeds_tamper\Form\TamperEditForm::checkAccess'
   

   I tested with an user that didn't have the administer feeds_tamper permission, but did have 'Tamper My importer feed type' and got an access denied. So the custom access check should check if the user has one of both permissions. Now both are required.

2. When I try to edit a Tamper in the UI, there is a select element to change the plugin to add, but when I select a different plugin, the form does not change. I think when editing an existing tamper, you should not be able to change the plugin. This is also how it works in D7.
3. In the D7 version of Feeds Tamper you were able to add an administrative description for the Tamper that you add. It would be useful to add a text field for that. I already defined the description key in feeds_tamper.schema.yml.

4. +++ b/src/FeedTypeTamperMetaInterface.php
   @@ -49,6 +49,18 @@ interface FeedTypeTamperMetaInterface extends ObjectWithPluginCollectionInterfac
      /**
   +   * Updates the tamper plugin instance.
   +   *
   +   * @param \Drupal\tamper\TamperInterface $tamper
   +   *   The tamper plugin instance
   +   * @param array $configuration
   +   *   An array of tamper configuration.
   +   *
   +   * @return $this
   +   */
   +  public function updateTamper(TamperInterface $tamper, array $configuration);
   

   Minor: there should be a test for this newly added method in FeedTypeTamperMetaTest.

I like it that individual parts of the D7 code are removed. This way we can see better what would be left to port later on.

@max
I've made start with writing up a review (found only tiny things so far) but wasn't able to finish it this week due me feeling a bit ill the last few days. Hopefully I'll be able to finish up the review by Monday.

Committed #5 with a few minor changes (see interdiff).

Thanks Max!