Use django-apptemplates to specify which templates are being extended

[rlskoeser]

https://pypi.org/project/django-apptemplates/

Had some transitory errors that looked as if they could be circular or a wrong version of a template being extended with mezzanine pages (possibly complicated by the way mezzanine looks for templates for pages based on url and class). Update consults and base richtextpage to make it explicit which template (cdhweb or mezzanine) is being extended.

[rlskoeser]

django-apptemplates doesn't actually help us here because of where mezzanine looks for the templates (in the main template directory), so we can't reference them by app.