When a request contains an ETag value in its If-None-Match header, the backend
may send a request ID (from Rack) that does not correspond to a value in Peek's
Redis cache (because we aborted the Rails processing in our ETag middleware).

Because a cached response (304) has to replace the headers with those from the
previous 200 - https://tools.ietf.org/html/rfc7234#section-4.3.4 - we add a
custom header that will only be present in cache hits, that can tell the
frontend to ignore these.

Closes #45412

[ci skip] Adds specs

Adds specs

Closes #45326

Revert "Merge branch '44870-remove-extra-space-around-comment-form-on-merge-requests' into 'master'"

This reverts merge request !18120

This prevents performing the requests, and disables all emoji reaction buttons

Includes:
- Model, factories, create service and controller actions
- As usual, includes specs for everything
- Builds UI (copy from PAT)
- Add revoke action

Closes #31591