Add an option to cache by album

[dsschult]

Add an option to do extended caching by album, instead of globally for the whole gallery.

This compliments #511 by not needing to load and save a huge pickle file every time, saving multiple seconds in my case. Only the album that gets updated has it's cache loaded and saved.

By default the old mode (global) is used, so it is opt-in.

I also made some variable names more consistent (removing camel case), since I was editing most of the file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (27e10d4) 88.72% compared to head (2b2d6e0) 89.40%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #512      +/-   ##
==========================================
+ Coverage   88.72%   89.40%   +0.68%     
==========================================
  Files          25       25              
  Lines        2066     2096      +30     
==========================================
+ Hits         1833     1874      +41     
+ Misses        233      222      -11     

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