Allow customisation of app's home path

HMRC contacts app puts its homepage within an admin namespace, so the crown link in the head needs to be modifiable.

* Introduce an `app_home_path` variable
* Default it to `root_path` for backwards compatibility
It's setup in `spec/dummy/app/views/layouts/application.html.erb`, a dummy app.
If I remember correctly, modifying a `content_for` on the fly then re-rendering the page to test for it is non-trivial.