[FEATURE] Use django_oauth_tools to implement better auth for Portal API

[swainn]

Is your feature request related to a problem? Please describe.
The Tethys Portal API was put together haphazardly to support the experimental React template and is using simple session auth to verify authentication:

tethys/tethys_portal/views/api.py

Line 25 in 9f5b60c

if not request.user.is_authenticated:

This works ok, but it would be better and probably more secure to implement an oauth/token pattern. This would also support our use case of the API for federating Portals in the future.

Describe the solution you'd like
Use django_oauth_tools to implement better auth for Portal API: https://django-oauth-toolkit.readthedocs.io/en/latest/rest-framework/getting_started.html#step-2-create-a-simple-api

Describe alternatives you've considered
N/A

Additional context
N/A