We call `MergeRequest#check_mergeability` on MR page load to trigger
mergeability check. Other than that, we rely on calls for
`MergeRequest#mergeable?` to eventually call it as well. This can
cause stale `MergeRequest#merge_status` when:

1. Race condition occurred between the MR page load and the time
   `NewMergeRequestWorker` runs.
2. The `MergeRequest#merge_status` gets updated after the MR page
   has already loaded (e.g. new changes were pushed to the MR's
   target branch).

As a fix, we are calling `MergeRequest#check_mergeability` in more
places to ensure we check for it:

1. We now call it on `MergeRequests::AfterCreateService` to prevent
   race condition issue. This service is called by the
   `NewMergeRequestWorker`. This way, we ensure that we check for
   MR's mergeability after it was created.
2. `MergeRequestPollCachedWidgetEntity#merge_status` has been
   updated so whenever the `MergeRequest#merge_status` gets
   updated and the MR widget polls for updated status, we will
   recheck for mergeability as well. On next poll, it should show
   the updated `merge_status`.
3. The `check_mergeability_async_in_widget` FF has also been
   removed since it doesn't seem to be used at all. We also need
   to keep the call for `MergeRequest#check_mergeability` on the
   show action so we won't need to wait for poll in case the merge
   status gets updated and the user decides to refresh the page.

Changelog: fixed