Add feature flags for each Rapid Diffs page

What does this MR do and why?

This MR puts every Rapid Diffs page behind a corresponding feature flag.

Page	Feature flag	rapid_diffs=true required?
New merge request	rapid_diffs_on_mr_creation	No ❌
Merge request changes	rapid_diffs_on_mr_show	Yes ✅
Commit view	rapid_diffs_on_commit_show	Yes ✅
Compare revisions	rapid_diffs_on_compare_show	Yes ✅

It also allows to use Rapid Diffs on merge request creation page without using rapid_diffs=true request parameter. Other pages are only accessible with the provided rapid_diffs=true query (and enabled feature flag for a specific page).

We're removing URL param requirement from the MR creation page because it will be the first page to support Rapid Diffs, which we plan to release in production soon. Two flags will be turned on in production: rapid_diffs and rapid_diffs_on_mr_creation.

Other pages require URL param because they are not planned to release yet and a URL param offers a quick way to toggle Rapid Diffs and compare them with current diffs.

Implements #534897 (closed), related to #534896 (closed).

How to set up and validate locally

1. Enable rapid_diffs feature flag
2. Enable rapid_diffs_on_mr_creation feature flag
3. Visit merge request creation page
4. Select source branch
5. Click 'Compare branches and continue'
6. Select 'Changes' tab
7. Observe Rapid Diffs to load