Support users in multiple review groups in API responses

Review Request #8458 — Created Oct. 12, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
4d6411e...

Reviewers

The patch that fixed the review group user resource links broke when a
user was a member of more than one group (which is a likely occurance).
This was because we were using a model_parent_key that was a many-to-
many field (and therefore the reverse relation was not unique),
resulting in a MultipleObjectsReturned exception. This has been fixed
by customizing how we retrieve the parent object IDs.

Ran unit tests.