Add a trivial_publish capability

Review Request #7681 — Created Oct. 7, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x

Reviewers

Since not all versions of Review Board will support trivial publishes,
it is a good idea to expose this is a server capability. That way,
rbtools and other consumers of the API can determine if publishing
(e.g., with rbt publish) can be done trivially.