Port system_settings_form to ConfigFormBase

Review of #8.
Please include an interdiff between patches. Very difficult to see what changed between patches this way... I guess most of the remarks of #8 still apply.

1. +++ b/src/Form/hypothesisForm.php
   @@ -0,0 +1,173 @@
   + * Class Settings
   

   Not according to coding standards.

2. +++ b/src/Form/hypothesisForm.php
   @@ -0,0 +1,173 @@
   +  public function buildForm(array $form, FormStateInterface $form_state)
   

   Missing PHPDoc.

3. +++ b/src/Form/hypothesisForm.php
   @@ -0,0 +1,173 @@
   +    // Default settings
   

   Comments should be closed with a full stop. (.)

4. +++ b/src/Form/hypothesisForm.php
   @@ -0,0 +1,173 @@
   +      '#title' => $this->t('Defaults Settings'),
   

   "Defaults Settings"? Should this be Default without the extra 's'?.

5. +++ b/src/Form/hypothesisForm.php
   @@ -0,0 +1,173 @@
   +      '#title' => $this->t('Highlights on by default'),
   

   Not really descriptive on it's own I think. Could be improved and also maybe add a description if you cant make it descriptive enough with the title alone.

6. +++ b/src/Form/hypothesisForm.php
   @@ -0,0 +1,173 @@
   +    // Visibility settings
   

   Missing full stop.

7. +++ b/src/Form/hypothesisForm.php
   @@ -0,0 +1,173 @@
   +    $hypothesis_visibility = $config->get('visibility');
   

   Should this be hypotesis_visibility ?

8. +++ b/src/Form/hypothesisForm.php
   @@ -0,0 +1,173 @@
   +      '#title' => $this->t('Show hypothesis on specific pages'),
   

   Be consistent with writing the module name. Should be with capital.

9. +++ b/src/Form/hypothesisForm.php
   @@ -0,0 +1,173 @@
   +      '#title' => '<span class="element-invisible">' . $title . '</span>',
   

   Is this the correct way to hide a title?

10. +++ b/src/Form/hypothesisForm.php
    @@ -0,0 +1,173 @@
    +      '#title' => $this->t('Show hypothesis for specific content types'),
    ...
    +      '#title' => $this->t('Show hypothesis for specific roles'),
    

    Module name with capital.

11. +++ b/src/Form/hypothesisForm.php
    @@ -0,0 +1,173 @@
    +      '#description' => $this->t('Show hypothesis only for the selected role(s). If you select no roles, hypothesis will be visible to all users.'),
    

    Module with capital.

12. +++ b/src/Form/hypothesisForm.php
    @@ -0,0 +1,173 @@
    +  public function validateForm(array &$form, FormStateInterface $form_state)
    

    No need to include this function if you only call the parent. Normally this is done automatically by doing the inherit.

13. +++ b/src/Form/hypothesisForm.php
    @@ -0,0 +1,173 @@
    +//
    

    Delete.

14. +++ b/src/Form/hypothesisForm.php
    @@ -0,0 +1,173 @@
    +    $config->set('pages', $form_state->getValue('hypothesis_pages'));
    

    would be better if you would prefix all the config values with the module I think. e.g. hypothesis_pages instead of pages.

Code wise it looks ok. Functionality wise I didn't test this.

Merged to the dev branch.