replace old add methods with unified logic
shared formatting/templating logic
restore correct formatting of paths

- don't sanitize paths (this is already done separately)
- album.path (or album['path']) is now an alias for album.item_dir(), which
  restores the formatting of $path in templates
fix tests for new path formatting

Slowly expunging the terrible idea that was `pathmod`...
expunge pathmod parameter

I started using this a long time ago as a misguided attempt to make
path-related functions more testable. Harnessing is better.
remove vestigial plugin hooks

These have been superseded by the new function-based approach.