TemplateHook has always required name, template_name, apply_to,

and extra_context to be passed into __init__(), even in a subclass.

This makes usage a bit verbose.
This change adds new class-level control of these options. Now, a

subclass can define these as class attributes and they'll be used if the

caller doesn't override any of them with a parameter.
Subclasses can also opt out of rendering by overriding

should_render(), which means they no longer need to handle this in a

template.

Unit tests pass.
Tested this with the banner component work in Review Board, utilizing

the new features.