Add default_auto_field and migration.

[valberg]

Fixes #202

[codecov]

Codecov Report

Merging #211 (2abc493) into master (2dd842e) will increase coverage by 0.53%.
The diff coverage is 100.00%.

❗ Current head 2abc493 differs from pull request most recent head b12d2f2. Consider uploading reports for the commit b12d2f2 to get more accurate results

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
+ Coverage   63.81%   64.34%   +0.53%     
==========================================
  Files          17       18       +1     
  Lines         467      474       +7     
==========================================
+ Hits          298      305       +7     
  Misses        169      169              

Impacted Files	Coverage Δ
invitations/apps.py	100.00% <100.00%> (ø)
invitations/migrations/0004_auto_20230328_1430.py	100.00% <100.00%> (ø)

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