Refactor controller methods

[JWCook]

In exchange for more code/doc duplication, I think this makes controller methods simpler and easier to follow. Or, rather, I think my previous attempts at code reuse may have hurt readability a bit.

This also adds several previously missing docstrings.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (a086ac6) 96.35% compared to head (5d00c3a) 95.96%.
Report is 6 commits behind head on main.

Files	Patch %	Lines
...yinaturalist/controllers/observation_controller.py	93.93%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #529      +/-   ##
==========================================
- Coverage   96.35%   95.96%   -0.39%     
==========================================
  Files          58       58              
  Lines        3294     3298       +4     
  Branches      643      635       -8     
==========================================
- Hits         3174     3165       -9     
- Misses         88       95       +7     
- Partials       32       38       +6     

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