Previously we used custom fields to denote a user was anonymous, this was
risky in that custom fields are prone to race conditions and are not
properly dedicated, missing constraints and so on.
The new table `anonymous_users` is properly protected. There is only one
possible shadow account per user, which is enforced using a constraint.
Every anonymous user will have a unique row in the new table.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Migrates email user options to a new data structure, where `email_always`, `email_direct` and `email_private_messages` are replace by
* `email_messages_level`, with options: `always`, `only_when_away` and `never` (defaults to `always`)
* `email_level`, with options: `always`, `only_when_away` and `never` (defaults to `only_when_away`)
As it stands we load up user records quite frequently on the topic pages,
this in turn pulls all the columns for the users being selected, just to
discard them after they are loaded
New structure keeps all options in a discrete table, this is better organised
and allows us to easily add more column without worrying about bloating the
user table
