pull-request: extended default reviewers functionality.

- added mandatory reviewers JS logic Fixes: #4219
- added option to forbid author to be a reviewer via JS/Frontend checks. Fixes #5190
- added option to forbid adding new reviewers except those defined. Fixes #5208

rhodecode/api/tests/test_get_pull_request.py

@@ -109,7 +109,8 @@ class TestGetPullRequest(object):
                     'reasons': reasons,
                     'review_status': st[0][1].status if st else 'not_reviewed',
                 }
-                for reviewer, reasons, st in pull_request.reviewers_statuses()
+                for reviewer, reasons, mandatory, st in
+                pull_request.reviewers_statuses()
             ]
         }
         assert_ok(id_, expected, response.body)