ETag caching for merge request notes polling
Description
In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9036 we introduced ETag caching for issues notes polling. The results of the experiment are in https://gitlab.com/gitlab-org/gitlab-ce/issues/29777. We can apply the same mechanism for notes polling for other resources. The level of difficulty is different for every resource so there's a separate issue for every resource.
Proposal
Enable ETag caching for merge request notes polling.
Gotchas
On a merge request page we also display notes from related commits (MergeRequest#related_commits). This makes cache invalidation harder, because we don't have an easy way to find all merge requests that contain a given commit.