Add logic to handle cancelled Apple receipts

[cdriehuys]

Closes #474

Proposed Changes

Receipts cancelled by Apple customer support are now correctly handled. Attempting to upload a receipt that has been cancelled will now reject the upload, and cancelled receipts are deleted by the background job used to update subscriptions.

[codecov[bot]]

Codecov Report

Merging #506 into master will increase coverage by 0.47%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #506      +/-   ##
==========================================
+ Coverage   97.82%   98.29%   +0.47%     
==========================================
  Files          60       60              
  Lines        1608     2054     +446     
  Branches       90      135      +45     
==========================================
+ Hits         1573     2019     +446     
  Misses         25       25              
  Partials       10       10

Impacted Files	Coverage Δ
...know_me/management/commands/updatesubscriptions.py	100% <100%> (ø)	:arrow_up:
km_api/know_me/subscriptions.py	100% <100%> (ø)	:arrow_up:
km_api/know_me/factories.py	100% <0%> (ø)	:arrow_up:
km_api/know_me/models.py	100% <0%> (ø)	:arrow_up:
km_api/km_api/urls.py	100% <0%> (ø)	:arrow_up:
...pi/know_me/serializers/subscription_serializers.py	100% <0%> (ø)	:arrow_up:
km_api/know_me/views.py	100% <0%> (ø)	:arrow_up:
km_api/know_me/admin.py	100% <0%> (ø)	:arrow_up:
km_api/know_me/serializers/__init__.py	100% <0%> (ø)	:arrow_up:
... and 1 more

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