Relational API links

- Adds a self-relational link to both individual API entities and
  collections.
- Adds null previous/next links to collections to support pagination in
  the future.
- Temporarily disables pagination (fixes bug 1079661), as the client
  doesn't currently support or expect it. We probably need to return all
  results by default unless the client requests a limited resultset
  anyway.
- Expands test coverage for granting roles to users & groups in projects
  & domains.

Change-Id: Ib5d6d39f5e1eb673c3285ef0b98603c5d375de75
Merge "Relational API links"