Add CMS_PAGE_TEMPLATE variable

This adds CMS_PAGE_TEMPLATE variable to the template context and fixes a bit of documentation in app_integration.rst

The same result can be achieved without defining a new variable by using `{{ request.current_page.get_template }}` but it looks cumbersome to me, and that method isn't documented anywhere, so it's not a public API.

This allows for something like `{% extends CMS_PAGE_TEMPLATE %}` in apphoked applications templates to dynamically inherit from the CMS page template