Add the ability to limit pull request notifications on a branch level instead of a repository level

Current behavior

currently, we can only watch or stop watching Pull Requests to set the notifications we will receive

Suggested Solution

It would be great if we could limit the pull request notifications on a branch level instead of a repository level,

Business case:

Some teams need a notification email to be sent when a pull request is merged to master and does not need a notification when a pull request is merged to other branches of the same repo