Improve the `Member` API entity to take actual `Member` objects instead of `User`
As noted in several comments:
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4833#note_13106456
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4833#note_13106472
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4833#note_13684168
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5599/diffs#note_13635017
This is weird that the Member
API entity takes a User
object rather than a Member
object. That being said, from a pragmatic point of view, we're actually exposing more user attributes than member attributes in this entity so that's probably why the current implementation is like it is. :)