Added the ability to delete your account.

You can now delete your account via the "edit profile" page. Since
instead of deactivating I have decided to have the account permanently
deleted from the table, I have removed the `users`.`date_deactivated`
column. The code does not delete posts or follows or other data
associated with the user, but once foreign keys have been established
(in a future commit) it will cascade the delete through the rest of the
tables so there is no residual data from deleting users.