N+1 queries for `user_notes_count` in Issues / MergeRequests API

The notes count is already computed in IssuableMetadata (https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/issuable_metadata.rb#L22) together with the downvotes and upvotes to prevent N+1 queries.

But this count is not being used in the IssueBasic and MergeRequestBasic entities. The code here should be similar to the upvotes / downvotes where it gets the data from issuable_metadata if available.