Keep finished pickup dates in DB

[tiltec]

So that the planned Feedback model can reference them.

Closes #328

I thought about writing a data migration that recovers deleted pickup dates from history entries, but I don't see the use case. I think it's sufficient if giving feedback is only possible for pickups that happened after this PR is merged. Opinions?

[codecov-io]

Codecov Report

Merging #344 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
+ Coverage   98.21%   98.22%   +<.01%     
==========================================
  Files         153      154       +1     
  Lines        4157     4165       +8     
  Branches      181      181              
==========================================
+ Hits         4083     4091       +8     
  Misses         54       54              
  Partials       20       20

Impacted Files	Coverage Δ
...s/migrations/0016_pickupdate_done_and_processed.py	100% <100%> (ø)
...odsaving/management/commands/create_sample_data.py	100% <100%> (ø)	⬆️
foodsaving/history/tests/test_api.py	100% <100%> (ø)	⬆️
...gement/tests/test_process_finished_pickup_dates.py	100% <100%> (ø)
foodsaving/stores/models.py	96% <100%> (+0.12%)	⬆️
...nagement/commands/process_finished_pickup_dates.py	100% <100%> (ø)

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