Integrate the frontend with the `single_mr_diff_view` feature flag

What does this MR do?

This is a partial implementation (backstage) for #33183 (closed).

The vast majority of these changes are simply to create a pipeline for the single_mr_diff_view feature flag value to be passed from the Rails controller rendering the MR page all the way into the Vue app and then through that and into the Vuex state.

There are a few changes to match the way we test for the feature flag. Namely, by using the project as the second parameter when checking the feature flag in diff_file_entity.rb.

Screenshots

There are no screenshots, as this is entirely ~backstage work.

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

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