Update the installer templates.

Now refer to the Taverna Player Workflow model.
Refactor ModelProxy so constantization is delayed.

Doing the constantize step in the initializer prevents us from setting up a
proxy before any models are actually loaded so delay it until first access.
Set the internal Workflow model to be the default.
Only route TavernaPlayer::Workflow if it is in use.

The host app can override this model so we don't always need to add the route.
Allow extending of the internal Workflow model.

Turn it into a concern like the other models.
Allow extending of the internal Workflow controller.

It is now a concern like the other controllers.
Add the internal Workflow stubs to the generators.
Set the workflow model proxy more simply.

If the standard overrides are suitable, otherwise continue to use the block
version.