Add end date to pickups

[nicksellen]

As part of karrot-dev/karrot-frontend#709

[codecov]

Codecov Report

Merging #622 into master will increase coverage by 0.03%.
The diff coverage is 98.98%.

@@            Coverage Diff             @@
##           master     #622      +/-   ##
==========================================
+ Coverage   95.77%   95.81%   +0.03%     
==========================================
  Files         346      351       +5     
  Lines       13573    13767     +194     
  Branches      849      854       +5     
==========================================
+ Hits        13000    13191     +191     
- Misses        457      461       +4     
+ Partials      116      115       -1

Impacted Files	Coverage Δ
foodsaving/conversations/emails.py	93.16% <ø> (ø)	⬆️
foodsaving/groups/models.py	98.18% <ø> (ø)	⬆️
foodsaving/notifications/tasks.py	100% <ø> (ø)	⬆️
foodsaving/pickups/receivers.py	95.23% <ø> (ø)	⬆️
foodsaving/subscriptions/tasks.py	89.04% <ø> (ø)	⬆️
foodsaving/groups/emails.py	95% <ø> (ø)	⬆️
foodsaving/base/filters.py	100% <100%> (ø)	⬆️
foodsaving/pickups/tests/test_filters.py	100% <100%> (ø)
foodsaving/userauth/tests/test_userauth_api.py	100% <100%> (ø)	⬆️
...saving/pickups/tests/test_pickupdate_series_api.py	99.44% <100%> (ø)	⬆️
... and 38 more

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[nicksellen]

Ok, it works! Although still some more thinking about how to make working with these ranges a bit nicer. I just put a lot of date_range(date, minutes=30) everywhere....

And probably some missing tests to see how updating it to have a nice end date works, etc...

... and then also we realised that part of the use case of having end dates is so that you could have very looooooooooooooooooooooooooooooooong pickups where you can do the pickup at any point, but in that case the useful thing is to be able to mark it as done part way through.

The idea would be to add a done field to pickups, and make our existing mechanisms use that too to check for doneness (as opposed to just checking if the date passed and it had collectors), but that should probably be for another PR.

So my goal to make this one mergable is to allow the existing frontend to work with minimal changes (but not bother about setting custom end dates - always have them 30 minutes).