Document how to migrate from a built-in User model to a custom User model

[Carl Meyer]

(I'd have thought we might already have a ticket for this, but I couldn't find it.)

So far our answer here has been "sorry, you can't do it, unless you're very familiar with the depths of the migrations system and willing to put a lot of time into it."

I don't believe that this is a tenable or defensible answer. It puts too many of our users, too frequently, into an impossible quandary. I think we need to clearly document how you can do it today, even if the process is nasty and ugly. Hopefully seeing that nasty and ugly documentation might clarify what could be improved to make the process less nasty and ugly.