Review Board's "account pages", which are Djblets ConfigPages with

dynamically determined forms have been ported to Djblets. The behaviour

for dynamically configuring a form is encapsulated in the

DynamicConfigPageMixin and the ConfigPageRegistry. Config pages

that are to be dynamic should inherit from the DynamicConfigPageMixin

and are to be managed by a single registry; multiple registries can

manage multiple sets of configuration pages, however.

Ran unit tests.
Used this with an upcoming change for Review Board.