Review Board is gaining a centralized ReviewRequestConditionsField

and a central registry that reflects any choices provided by extensions.

This change makes use of that, providing a compatibility class for older

versions of Review Board that behaves as before.
This change pretty much copies /r/14281 where we enabled extensible

condition support for RBIntegrations.

Ran unit tests.
Saw Power Pack's new owner and participant user role conditions

  in a tool config page.
Was able to trigger the tool using the owner condition (the

  participant condition requires more work in a separate change).
Tested on RB 7.0.4 dev server too.