add support for Django web frontend

[tomkralidis]

Is your feature request related to a problem? Please describe.
pygeoapi should provide a Django front end for downsteam applications to leverage plug and play OGC API functionality. This ticket is in context of supporting pygeoapi as a possible backend for GeoNode, as presented at the GeoNode Summit 2020.

Describe the solution you'd like

• supports a --django flag when running pygeoapi start
• directory layout:

django_app/
    urls.py  # routes
    views.py  # binding
    settings.py  # configuration

• the app should be reusable by downstream Django applications such as GeoNode

Describe alternatives you've considered
None yet

Additional context

• proof of concept in Dominode: Integrate pygeoapi dominodeorg/dominode#120

[francbartoli]

Closed by #630