Since https://reviews.reviewboard.org/r/9131/ did not make it into the

release window for RB 3.0, we cannot address the issues in the original

implementation of class-based actions. So we are gating them behind a

feature so they can be marked as experimental (and not to be used in

production). That way we will be able to fix up the issues in the

implementation for a future release.
This patch also addresses the issue of the new action hooks not

unregistering their child actions.
Documentation on action hooks has been reverted to how it was in

Review Board 2.5.

Unit tests all pass.
Wrote a test extension which (1) registered a hook that uses

  class-based actions and (2) registered a hook using dict-based

  actions. The former were not available (and was logged as such) and

  the latter were.