Add support for multiple linked_orderpositions on reusable media

[wiffbi]

This PR adds a ManyToManyField on ReusableMedium for linked_orderpositions. It keeps linked_orderposition in the API for backwards-compatibility. If the medium has more than one linked_orderposition, then the API does not output property linked_orderposition for that medium and blocks PATCH requests with linked_orderposition as invalid.

It also adds a field secret to the medium, so in the future, the medium can be claimed by a customer through the identifier and secret.

internal note: update & merge pretixpos-android!361 after merging this pr

[codecov]

Codecov Report

❌ Patch coverage is 91.95402% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.57%. Comparing base (da43984) to head (1ca3ef1).
⚠️ Report is 69 commits behind head on master.

Files with missing lines	Patch %	Lines
src/pretix/api/serializers/order.py	87.50%	3 Missing ⚠️
src/pretix/api/serializers/media.py	95.45%	1 Missing ⚠️
src/pretix/api/views/checkin.py	90.90%	1 Missing ⚠️
src/pretix/base/exporters/reusablemedia.py	66.66%	1 Missing ⚠️
src/pretix/control/forms/organizer.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5666      +/-   ##
==========================================
+ Coverage   78.55%   78.57%   +0.02%     
==========================================
  Files         471      472       +1     
  Lines       67755    67883     +128     
==========================================
+ Hits        53223    53341     +118     
- Misses      14532    14542      +10     

Files with missing lines	Coverage Δ
src/pretix/api/views/media.py	97.97% <100.00%> (ø)
src/pretix/api/views/order.py	82.67% <100.00%> (+0.05%)	⬆️
src/pretix/base/models/media.py	98.27% <100.00%> (+1.84%)	⬆️
src/pretix/base/services/orders.py	88.54% <100.00%> (+0.05%)	⬆️
src/pretix/control/forms/filter.py	70.47% <ø> (ø)
src/pretix/control/logdisplay.py	62.84% <ø> (ø)
src/pretix/control/views/organizer.py	78.29% <100.00%> (-0.03%)	⬇️
src/pretix/api/serializers/media.py	97.14% <95.45%> (-1.01%)	⬇️
src/pretix/api/views/checkin.py	90.98% <90.90%> (+0.09%)	⬆️
src/pretix/base/exporters/reusablemedia.py	72.00% <66.66%> (+4.00%)	⬆️
... and 2 more

... and 27 files with indirect coverage changes

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[robbi5]

Currently, when editing a reusable medium, the order code 3UYXK and position -1 is only shown for things I'm adding fresh to the select. The already linked tickets are only displayed with their position, but the order code would be great to know to differentiate them :)

[wiffbi]

Currently, when editing a reusable medium, the order code 3UYXK and position -1 is only shown for things I'm adding fresh to the select. The already linked tickets are only displayed with their position, but the order code would be great to know to differentiate them :)

Good idea, when using it as a single-choice field, it already had a label_from_instance that does this. Added this to the multiple-choice version as well.