ENHANCEMENT: Preserve URL History

Ensure a record for each unique URL a page has had remains in the DB so
automatic 301 redirects continue to work
Separate from onAfterWrite

Separate functionality so it can be called independently of a write()
e.g. for use in Tasks
NEW: Truncate All and Remove Deleted Tasks

Added build tasks for truncating all site tree objects at once (e.g.
after the first time you install the module), and for completely
removing deleted pages from the database