ETag caching for commit 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 commit notes polling.

Gotchas

Commit is not an ActiveRecord model.