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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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