Mechanism for handling deactivated users

[simonw]

For Datasette Cloud (and self-hosted Datasette too) we need a way to mark a user as "inactive" - meaning they are no longer a part of the space, but we still want to store their job title / display name / avatar so we can display it in places like datasette-comments against their old content.

This should impact if they are shown in the list of profiles and also how they integrate with things like autocomplete features elsewhere.