FIX: Ensure that actions inferred from templates with the "_action" suff...

...ix also respect allowed_actions.

FIX: Ensure SSViewer::hasTemplate() is aware of themes.

To do this, RequestHandler::definingClassForAction() has been created, splitting out the code that looks up the class that defines a given action into its own method.  This is then overridden in Controller to look at templates.