Finer-grained, easier-to-filter email notifications (BB-9467)

My team use bitbucket extensively but, as an admin, I find that I get a lot of traffic from bitbucket and it's hard to pick out the messages that I really care about with filter rules in my email client. I'd like to be able to distinguish the following traffic:

• pull request traffic for pull requests that I initiated
• important messages such as "approved" or "declined"
• "new pull request" messages where I'm one of the reviewers or where I'm added as a reviewer to an existing pull request
• subsequent traffic for pull requests for which I'm a reviewer
• pull request traffic for pull requests that I'm not a reviewer (I want to file these for occasional review rather than have them drop into my inbox)

The big problem I have right now is I can't filter the messages because each individual message doesn't seem to have the relevant context in it. For example, a comment on a pull request has the name of the pull request in it but it doesn't have a line saying "this is your pull request" or "you are a reviewer on this pull request" for me to filter on.