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/

/cc @JobV @dzaporozhets

From implementation perspective I can see it as separate page where we list row per subscription. Subscription can be either mailing list or individual user. User can only subscribe/unsubscribe with specifying filter, daily etc. Project master can add/remove any subscription.

I am always sceptical to features that I don't use so I would like to see some upvotes from other people here before we consider all 4 points.

Added ~139512 label

Added ~139607 label

@dzaporozhets interesting, thanks for the proposal

mentioned in commit 3b50e79f

Added usability label

I would at least like to see 1 through 3; these are features that I really do appreciate in Phabricator's Herald application.

Mentioned in commit 3b50e79f

We just started using GitLab EE where I work and I'm getting a lot of complaints from users about how many emails they get. They would really like to be able to receive a digest email once a day or so. So I'm on number 4.

added Accepting Merge Requests label