With the new action framework changes that separate out action model

registration from rendering, we were now registering every single action

on every page, whether or not they were being used. This meant that any

user would end up seeing actions for other pages, including the admin

page, which certainly wouldn't be correct. While there are cases where

an action may be worth registering when not rendered, in most cases we

want to avoid this behavior.
This change tackles this in two ways:


All rendered action IDs are tracked in the PageState, making it

   easy to determine which actions have been placed on the page in some

   way.


A new BaseAction.should_register() has been added that returns

   whether registration of the model should take place. By default,

   this will only return True if the action has been rendered (which

   has the benefit of inheriting the result of should_render()).

   Subclasses can override this to always register, or to limit to

   certain URLs or other criteria (but should always respect

should_render()).


With this, actions are now only registered on the pages in which they're

used, by default, without limiting more advanced use cases.

Unit tests pass.
Verified that admin actions weren't showing up on other pages, but were

showing up in the admin UI.