Break out notification-styling into type-dependent reusable components

[hmpf]

There are multiple ways of sending SMSes: SaaSes like Twilio, talking to a locally connected phone etc. The style of the content is the same however, 160 characters. That styling can possibly be reused for slack.

For email, Django has multiple backends, so we hopefully can use that instead of our own plugin system. But: the styling for subject and body can be reused for Teams channels.

[hmpf]

See #121.
Also, this is a requirement for #107