Administration and presets of notifications

**Is your feature request related to a problem? If so, please describe.**

New user by default do not watch any projects, they have to activate this feature manually before they receive email notifications e.g. for new strings or components.
This leads to a lot of users that register once, forget about the project and never come back. Having the monthly digest for notification of new components auto-enabled for every users would help me engange my idle users, I guess.
As an admin, I would like to be able to enable the notification for monthly digest for new component for all my users, or for specific users, just as I can add them to a group.
Also, I would like to change the presets for notifications for newly registering users. I would like that when a user registers, the notification for "monthly digest for new component" should be auto-on by default.

**Describe the solution you'd like**
When I am logged in as Admin/Superuser, there should be a tab in Administration -> Users or in the django admin interface, that allows me to edit the notification presets for new users, and/or to administrate/override single user/group notification settings.

**Describe alternatives you've considered**

I tried to find the presets in the django/accounts module and in the database, I thought they must exist somewhere, at least hard-coded but I failed. If you don't plan to implement this feature, I would be thankful for pointers where I can find these presets, or where the notification settings are stored.