This adds the ability to ban a user by their IP address.
When an admin is viewing a user profile, a new "Ban user" button is
present. Clicking on that will:
- Create a ban on the IP addresses that were tracked for that user's
sessions
- Remove all the messages authored by that user
- Log the user out immediately
In addition, that user will no longer be shown in most user lists in the
app. They are still shown to admins, in account settings. Viewing their
profile from there will now show a "Remove ban" button which can be used
to restore their access (it doesn't restore their messages though --
those are already gone -- it just removes the blocks so they can log in
again).
