When creating a new review request with no reviewer or groups

assigned, attempting to publish it with PUT will be sucessful.

However, it shouldn't allow that as review requests must have a

reviewer. The issue is solved by adding conditions to validate the

review draft on the server.

Added a unit test, which passes (along with all other tests).

Created a draft test review that don't have a reviewer/group name.
Ran it using this command ./tests/runtests.py 

  reviewboard.reviews.tests.test_review_request_draft 
This will raise a publishingError since there isn't reviewer.
Fixed other unit tests that failed because of the change.