Load improvements related to MR widget
What does this MR do?
- Stop using the
MergeRequestEntity
to load sidebar features (useMergeRequestBasicEntity
instead) - Drop
merge_check
endpoint (use onlyshow
action for load/polling) - Remove unnecessary initial request to load the widget (we were previously making a request to
merge_check
) - Set proper header to be used on FE polling
- Add query count tests
Related to gitlab-ee!1933
Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/32536
EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1935