Bug 824037: Initial infrastructure to bring back page deletion.

(don't merge this yet; notes below outline what still needs to be done, I just want to get eyes on it)

This replaces deletion with more of a "trash can" effect; a deleted
Document remains in the database, but is excluded from queries. A
separate "purge" method can actually permanently remove the Document.

Additionally, the deletion mechanism differentiates between actual
Documents, and redirects; redirects can always just be deleted in one
go, rather than lingering in the database.

Additional work still needed:

1. Code and UI-level support, when someone attempts to create a new
   Document with the same locale/slug as a deleted one, for informing
   the user what's going on and giving them the option to start from
   the last revision of the deleted Document or just start fresh.