Disabled 'Auto add reviewers' handler still fills in reviewers field in PR creation UI

The front-end auto-filling of the reviewers field is accomplished with the help of a REST endpoint which checks every configured 'Auto add reviewers' handler to find those that are applicable to the PR being created. However, the endpoint does not check whether a handler is disabled, and therefore whether it should refrain from instructing the front end to auto-fill the corresponding reviewers.