Source Editor: The toolbar integration

This is the last MR in the series aiming at implementing the toolbar MVC for the Source Editor:

• Source Editor toolbar feature flag
• Source Editor: The toolbar Vue components
• Source Editor: The toolbar integration (this MR)

What does this MR do?

• The usage of source_editor_toolbar feature flag scoped to a user: !87145 (4f526b37)
• Complete local state management for the toolbar using Apollo (we do not request any data from GraphQL for now):
  • !87145 (72278484), and
  • !87145 (d5078055)
• The Toolbar extension that we can use/unuse for any SE instance based on the context: !87145 (42eac4e0)
  • !87145 (03305aeb)
  • !87145 (ca4fe65e)
• The very first example of adding a button to the new toolbar. In this case ,the button is for toggling the live preview when editing a markdown file: !87145 (20d3b164)

How to test?

• In Rails console (rails c), enable the source_editor_toolbar feature flag for your user ('root' in the example below):

[1] pry(main)> Feature.enable(:source_editor_toolbar, User.find_by_username("root"))

• Go to a repository
• Pick an .md file in the repository and open it
• Click "Edit" to edit this markdown file
• You should notice the toolbar and the button as presented on the picture below

• You should be able to toggle the live preview by doing both:
  • toggle the button by clicking on it: toggles the preview panel and marks the button as active
  • 
  • toggle the contextual "Preview Markdown" action
  • 
  • In both cases, the result should be identical: the preview panel is toggled and the live-preview enabled/disabled

Related to #280800 (closed)