Give the Administrator of the repository the ability to remove watchers by issue (BB-13544)

This would complement issue BCLOUD-4992 "Add possibility of adding watchers to a ticket (other than one's self) (BB-6234)"

At our company, some inexperienced Bitbucket users often watch issues that aren't related to their "tasks" and are asking about the e-mail notifications they are getting. It would be very convenient for the admin to be able to fix this matter himself by removing specific watchers on specific issues.