Create a new Balrog notification scheme

[bhearsum]

Currently balrog send a email notification when a rule (and others entities) is inserted,updated and deleted. The logic behind this is closely related to the database layer flow, making it difficult to meet new notification requirements like as bug 1376537, bug 1337879, bug 1417589 and bug 1426221.

Talking to :bhearsum, I proposed create a Pulse based notifier (take look at attachment to better understand).

This bug intends move all work with emails to the new notifier and make all possible notifications requirements more flexible.

Ideas are welcome.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1452412, originally filed by @allan-silva)