Split accounts.forms into new modules.
While working on the new My Account page, I hit some circular dependency

issues with forms and backends. These weren't new, it turns out, as we

had sprinkled imports in the PreferencesForm functions.
Rather than adding more of that, I decided to split this file up.

There's now three modules: account (for account-related forms),

registration, and auth (for auth backends).

Unit tests pass.
Tested that the registration form still worked.
Tested that the My Account page still worked.
Tested that auth backends loaded and saved in the Auth Settings page.