Media refactor: prep for sending SMSes in addition to email

[hmpf]

Depends on #102 as well as #104.

[codecov-commenter]

Codecov Report

Merging #105 into master will decrease coverage by 0.66%.
The diff coverage is 43.29%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
- Coverage   70.12%   69.45%   -0.67%     
==========================================
  Files          34       36       +2     
  Lines        1339     1349      +10     
==========================================
- Hits          939      937       -2     
- Misses        400      412      +12     

Impacted Files	Coverage Δ
src/argus/notificationprofile/media/email.py	33.33% <33.33%> (ø)
src/argus/notificationprofile/media/__init__.py	41.17% <41.17%> (ø)
src/argus/notificationprofile/media/base.py	88.88% <88.88%> (ø)
src/argus/incident/views.py	70.75% <100.00%> (-0.28%)	⬇️
src/argus/site/utils.py	70.37% <0.00%> (-2.05%)	⬇️
src/argus/auth/views.py	81.25% <0.00%> (-1.11%)	⬇️
src/argus/incident/models.py	77.30% <0.00%> (-0.93%)	⬇️
src/argus/notificationprofile/views.py	70.96% <0.00%> (-0.91%)	⬇️
src/argus/incident/fields.py	89.18% <0.00%> (-0.29%)	⬇️
... and 4 more

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[hmpf]

I realized while writing the documentation that more refactoring is needed. Currently it is not the notification-medium that selects which email-address or phone number to use, that happens before we hit the class. I'm thinking that logic should be in the class. Not in the send()-method, but a new method that needs a notification profile. I'll push that documentation change anyway since it does describe the situation resulting from this and the previous PRs in this series.

[hmpf]

I've recorded some refactoring ideas in #121. I'm merging the current state in, so that there is a notification system.