Split Projects::MergeRequestsController#show?serializer=widget into two separate requests
Currently this widget serializer already contains too much information and it takes quite a lot of time to process this query. What's worse we run this request periodically by FE (to show up-to-date info on the MR show page w/o refreshing whole page). There is a related issue for using etag caching for this request - https://gitlab.com/gitlab-org/gitlab-ce/issues/61559. This issue is prerequisite to https://gitlab.com/gitlab-org/gitlab-ce/issues/61559 and the goal of this issue is:
- review which information from MR's widget serializer are being actually used to update MR's widget serializer asynchronously - maybe some of this information is not being updated in page?
- create new endpoint for periodic polling of the MR's widget serializer which will return only data really necessary for periodic update - in worst case it will be all existing information in the current widget serializer (but new endpoint is needed to avoid using
?serializer=widget
with etag caching because https://gitlab.com/gitlab-org/gitlab-ce/issues/58500