docs: Explain the difference between ADMIN groups
admin: Import exceptions from the same file

Use exceptions.py as the file to import admin exceptions from.
admin: Get subclasses of AdminJSONView properly

In the `views.py` of each resource, add a JSON_CLASS attribute that
holds the class that will be used for the /json/... requests.
admin: Reduce logging levels

Switch logging from info to debug in various places in the code. Also,
simplify the check for permitted user groups in the
`admin_user_required` decorator.