Toggle File-By-File setting from the MR settings dropdown

What does this MR do?

For #233898 (closed)

Adds a toggle that adds an override to the single file view user preference that is toggleable from any merge request changes tab.

This new toggle checkbox:

• updates the global store setting
• adds/modifies a URL search parameter
• adds/modifies a cookie

The diffs app is updated to rely primarily on the global store setting (which is initially set by the user preference, but then defers to the Vuex value).

The new URL search parameter and the cookie will temporarily change the user preference by overriding it in a sequence of priority:

1. User preference
2. Cookie
3. URL search parameter

When the cookie and the URL search parameter are removed, the user preference will again take precedence.

Review Notes

I suggest reviewing this commit-by-commit in chronological order.
Each portion of the changes is built up in the sequence of commits.

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team