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

As discussed in !1825 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.

See merge request !1867