This introduces support for an HTTP PUT to the User resource for

updating a full name, e-mail address, and the is_active flag. The full

name and e-mail address can be updated only if the auth backend supports

it, and if it does then the backend will be notified of any changes.
Users can modify only their own full name and e-mail address.

Administrators and special users with the auth.change_user permission

can also update these fields for other users, along with the is_active

flag.
There's also a fix for e-mail validation errors when creating a new user.
At this time, user settings and extra_data cannot be modified or

retrieved via the API, as we need to consider whether users should be

able to see this state for other users and whether there could be any

security impacts.

Unit tests pass.