Improve calendar and reservation API

[mahoyen]

Started on writing tests for the API, to make sure no functionality is broken before moving on to improving it.

Will maybe use Django Rest Framework to improve the API.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.55%. Comparing base (ae2d774) to head (d03c886).
⚠️ Report is 900 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #390      +/-   ##
==========================================
+ Coverage   83.71%   86.55%   +2.84%     
==========================================
  Files         118      258     +140     
  Lines        3837     9976    +6139     
==========================================
+ Hits         3212     8635    +5423     
- Misses        625     1341     +716     

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[ddabble]

Did an oopsie - see #758 for context :)