Improved email on push service

Right now we have email on push that lets you define a set of email addresses (commonly a mailing list) that gets emailed on every push.

I think emails on push are really relevant for large companies and we should look into:

1. Allowing people to sign up and unsubscribe themselves (currently only a project master can access the plugin)
2. Allowing regexp rules to filter for what files you want to receive emails
3. Allowing personal rules (instead of only project rules)
4. Allowing daily digest emails (instead of an email on every push)

Inspired by:

• https://secure.phabricator.com/book/phabricator/article/herald/
• http://cramer.io/2014/05/03/on-pull-requests/

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