Redo incoming email support

Incoming email support is too confusing. Because of the way this feature was implemented, there is a mail_room.yml inside of the CE repository that is used by source installations and in omnibus-gitlab we are re-creating that file from a template.

This means that the defaults in CE can diverge from defaults in omnibus-gitlab which is not good. It already caused confusion and prompted !810 (merged) .

We need to find a better way of handling this.

cc @DouweM @jacobvosmaer-gitlab @stanhu @twk3