Destroy talk media upon user deletion

[Splines]

Related to #647.

We noticed that destroying users by calling destroy directly (instead of calling the archive_and_destroy method) might result in invalid media as it has no editors anymore. From now on, we want to delete talk media of users when the respective user is destroyed under the following conditions:

• If called from archive_and_destroy -> STOP and don't do anything
• The user must not have any other media than talk media, e.g. the user must not have edited any lecture media.
  Otherwise -> STOP and don't do anything (*)
• A talk medium is only deleted if the user is the sole editor of that medium. That is, if there are multiple editors, we will just ignore that specific talk medium and try to delete the next one.

(*) "Don't do anything" is just a safety measure. In the UserCleaner in #647, those users are considered to be non-generic, so we shouldn't call their destroy method in the first place.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.90%. Comparing base (ed867fb) to head (eeebde2).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #651      +/-   ##
==========================================
+ Coverage   66.64%   66.90%   +0.26%     
==========================================
  Files         309      310       +1     
  Lines        9386     9440      +54     
==========================================
+ Hits         6255     6316      +61     
+ Misses       3131     3124       -7     

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[fosterfarrell9]

LGTM!