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
That's what `govuk_admin_template_engine_path` resolves to. See https://github.com/alphagov/govuk_admin_template/pull/56/files#diff-5d1d10abf0e5798555e15c414aa869e8R4