Allow deleting dbinbox account

This allows the user to delete their own dbinbox account.

* Disable the "delete" button until the user types "DELETE" to confirm
* First verify the user owns the same Dropbox account
* As far as I can tell, the Dropbox API doesn't allow one to remove your app from the user's Dropbox account. However, removing our serialized session ensures we can't actually access their account anymore.
* The username is open for re-registration by anybody.