admin: Add 405 page for json/... urls

The urls that start with "json/..." are intended only for use with the
Datatables API. If a user requests this page from his/her browser, throw
a 405 page.
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.