More fine-grain control over what notifications are received from watching pull requests (BB-15171)

BitBucket sends emails on pull request updates which a user is watching. The problem is that these pull-request-related-commit-notifications are just far too spammy. User should be able to enable or disable the notifications they want to receive regarding the pull-requests they are watching. For example a user just wants to receive the notifications when some other user comments on a pull request and not the commit notifications related to pull request.