This introduces a few changes to help write actions and ensure they work

in a consistent way in multiple places.
There's now an action_base.html template that handles the general

structure of an action. This provides the parent rb-c-actions__action

container with some consistent attributes, plus template variables for

use in derived templates. Derived templates can use this to build their

elements and to influence the parent container state.
This gives us more consistent templates and helps us set some new

attributes for visibility management. We use both hidden and

display: none (the latter being necessary since display can override

the former). From that, we can better manage the accessibility tree and

check for state more directly.
RB.ActionView provides the other end of the visibility management.

Visibility is now managed on the parent action container element, if

used, so that we don't end up with empty <li> elements affecting

layout when actions are hidden. If nested in another container (such as

a menu), the action's own element is used.
ActionView.show() and hide() update both display and hidden,

using the visibility element. An isVisible() method returns the state

based on that as well.
Along with this, there are a few small typing and function signature

fixes I hit during development, plus error handling when action

templates fail to render.
These will make it easier to implement some new action functionality

coming in a follow-up change.

Tested all the current actions in the UI, looking for visual breakages

or issues invoking the actions.
Unit tests pass.
Tested the new functionality with in-progress changes.