Add the ability to fetch the user categories for a book to proxy metadata. Add a formatter function to use it.
Revert "Add the ability to fetch the user categories for a book to proxy metadata. Add a formatter function to use it."

This reverts commit 29e29fdd3fb1d40fbb2398970b6be2360e52dc0e.
Alternate implementation that makes the decision about composite columns in user_categories_for_books().
Template language: New function user_categories() to return the list of User Categories for a given book (only works in the GUI).

Merge branch 'master' of https://github.com/cbhaley/calibre