When config pages were moved from Review Board to Djblets, the

ConfigPageForm.profile attribute remained. This was never used in

Djblets, and with User.get_profile() no longer being a thing in Django

itself, it doesn't make sense to have this here.
This is now deprecated. Instead of setting it automatically in the

constructor, it's now a dynamic property that warns when accessed.

Subclasses are advised to fetch their own concept of a profile

themselves.
A unit test was added to test the warning.

Unit tests pass.