Improve the Member API entity to take actual Member objects instead of User

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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. :)