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
I figured that bit out. My concern is that the test looks incomplete if there's not setup data corresponding to the asserted value. Where does `govuk_admin_template_engine_path` get set?