Retire `Bridgetown::URL`

[jaredcwhite]

This is a class that was used heavily by both Document and Page, but once we've migrated to the new resource content engine, it won't really be necessary any longer. (Page will move to GeneratedPage and ~~no longer need any automated permalink algorithm~~.)

UPDATE: actually GeneratedPage still uses the URL class heavily, so I need to unravel what it's getting used for exactly.