Return if evaluation was already published
Add a test case for publishing an already published evaluation
Merge branch 'master' into Issue1908
Fixed unpublished action and added test cases

Allows unpublish action to return immediately with status message to
user if evaluation is no longer published.
Added some tests for this.
Merge branch 'master' into Issue1908
[1908] Assertion failure in InstructorEvalPublishAction

fixes #1908
Merge branch 'master' of https://github.com/TEAMMATES/repo