Allow opt-out of specific email notification types

Right now the Stash notifications are very chatty: everyone on the PR gets an email for every comment and every new commit. The former is fine (although batching in STASH-2839 would be an improvement) but the latter are almost always just noise. For example:

• Author submits PR and tags two reviewers.
• Reviewers provide substantive feedback that requires the author to refactor a large part of the PR.
• Author goes through the process of commiting the new changes, iterating and pushing each commit as she goes (or as she tries to get the build to go green, for example).

Each of the additional commits sends an email to the reviewers, even though the PR may not be ready for re-review. Ideally, the reviewer should comment "Updated, please take a look" when the PR is ready for re-review, which would make the intermediate commit emails just noise.

I'd like to either kill these notifications globally or allow users to opt-out of them.