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
Changed internals so trust levels are referred to with
TrustLevel[1], TrustLevel[2] etc.
This gives us much better flexibility naming trust levels, these names
are meant to be controlled by various communities.
* rename `User#password_required` to `User#password_required!`
* emails with "i" @ something are a special case as well
* get rid of `self.` and returns where possible
* prefer "unless a" instead of "if !a"
* `unread_notifications` without manually iterating
* introduce `User#moderator?`
* introduce `TrustLevel#valid_key?`, `TrustLevel#compare`, and
`TrustLevel#level_key`
