Close conversations when target has ended

[tiltec]

Closes karrot-dev/karrot-frontend#1088

If the target has ended (e.g. pickups, applications, issues) and nobody replied within the last 7 days, close the discussion.
Nobody can write in a closed discussion. Includes handling for replies that are received via email, in this case it sends a "message not accepted" email back to the sender.

[codecov]

Codecov Report

Merging #635 into master will decrease coverage by 0.01%.
The diff coverage is 94.28%.

@@            Coverage Diff             @@
##           master     #635      +/-   ##
==========================================
- Coverage   95.79%   95.78%   -0.02%     
==========================================
  Files         359      362       +3     
  Lines       13842    13942     +100     
  Branches      854      858       +4     
==========================================
+ Hits        13260    13354      +94     
- Misses        464      470       +6     
  Partials      118      118

Impacted Files	Coverage Δ
foodsaving/subscriptions/tests/test_receivers.py	99.37% <ø> (ø)	⬆️
foodsaving/conversations/tests/test_api.py	100% <100%> (ø)	⬆️
...rsations/migrations/0027_conversation_is_closed.py	100% <100%> (ø)
foodsaving/conversations/tasks.py	96.15% <100%> (+0.91%)	⬆️
foodsaving/conversations/tests/test_tasks.py	99.17% <100%> (+0.19%)	⬆️
foodsaving/issues/factories.py	100% <100%> (ø)	⬆️
foodsaving/webhooks/stats.py	100% <100%> (ø)
foodsaving/webhooks/emails.py	100% <100%> (ø)
foodsaving/issues/models.py	97.74% <100%> (+0.03%)	⬆️
foodsaving/conversations/serializers.py	97.94% <100%> (+0.04%)	⬆️
... and 9 more

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