Add upvote/downvote fields to merge request and note API to preserve compatibility

As discussed in !1825 (merged) we should not break the API compatibility.

• This MR adds the fields upvotes/downvotes to the merge request API again, which always return 0.
• Add the fields upvote/downvote to the notes API, which always return false

This behavior is documented in the API docs.

This needs to go into 8.2 @rspeicher @vsizov