Bug 824037: Initial infrastructure to bring back page deletion.

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. UI for trusted users to be able to "purge".

2. 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.
Merge pull request #1293 from ubernostrum/page-deletion-824037

Bug 824037: Initial infrastructure to bring back page deletion.