Consolidate Edit and Web IDE buttons to single edit experience

Problem to Solve

The Web IDE is a more complete editing experience that helps to facilitate workflows across multiple files and merge requests. However, users favor the single file editing experience.

Additional Details

This is a test that should be setup behind a feature flag to see what kind/if any feedback is generated by making this change.

Proposal

We should consolidate the Edit and Web IDE buttons into a single split dropdown.

• On the File (/-/blob/branch/file) page replace the Edit and Web IDE button with a split dropdown
• Default button to Edit in Web IDE
• Clicking the attached dropdown button opens a dropdown with options to set the default button action
  • Edit in Web IDE
  • Edit file
  • Additional options
• Clicking the button should load the Web IDE/Single file editor/Other based on the chosen option
• Option choice should be persisted

Desc	Mock
Web IDE option chosen (default)
Edit option chosen
Dropdown
Dropdown with additional options

Note The above mockups are showing the future state of the dropdown component based on gitlab-design#1030 (closed)

Feature Flag

This feature needs to be done with a feature flag and it would be good to have the ability to either assign groups to the feature or assign individual users depending on what we want to do for testing.

We SHOULD NOT enable this by default in the release, but rather at a minimum toggle the feature flag on for gitlab-org and gitlab-com.

Instrumentation

It would be good as part of this to add telemetry to these buttons to see how many clicks each button receives: https://docs.gitlab.com/ee/development/telemetry/snowplow.html#implementing-snowplow-js-frontend-tracking

There is also a basic A/B test process documented here: https://docs.gitlab.com/ee/development/experiment_guide/#how-to-create-an-ab-test