Make Features UI theme-agnostic

Problem/Motivation

Claro recently got a bug report related to Features user interface #3067755: Boolean checkbox alignment on Features UI’s feature edit form. While trying to debug the issue we discovered that Features UI works only with Seven or the Seven-based Adminimal theme.

This issue is about the Drupal\features_ui\Form\FeaturesEditForm.

Proposed resolution

Refactor the features edit form and the related admin.css and Javascript to be theme-agnosting:

• Replace the (theme-provided) CSS classes that are used for JavaScript functionality with classes that are added by the form (or Drupal core).
• Replace the (theme-provided) CSS classes that are used for component theming classes that are added by the form (or Drupal core).
• Refactor the admin JS and CSS.

Remaining tasks

Must have: provide a patch.
Good-to-have: since the motivation is to make Features UI theme-agnostic, it would be a great idea to add a JavaScript test(s) for covering the UI’s functionality.

User interface changes

Nothing for Seven or Adminimal
I've just noticed that Features misses some RTL styles, so:

• No change expected for Seven or Adminimal themes with an LTR language
• Better RTL styles for Seven or Adminimal
• Acceptable look for other themes
• Fully functional JS

API changes

Nothing.

Data model changes

Nothing.

Release notes snippet

Not needed IMHO.