[DO NOT MERGE]: fixes #7734 - Add user mail preferences

This PR is based on PR #1813 and should be tested After #1813 is merged.
Adds the option for a user to set when he wishes to receive mails (daily, weekly, monthly).
I like this, we should take advantage of replacing the multiple_checkboxes to show the description here too.

That being said, not every notification has an interval. For example, "Puppet Error State" happens at the exact moment puppet sends a failure (see https://github.com/theforeman/foreman/pull/1856/files#diff-5f2522f8937b1076938bcca2ab931cf1R100).

A user should be able to subscribe to them, but it's not something we'd send recurringly.