Use @chained_auth_function with core auth functions

[alycejenni]

This is essentially an updated PR based on #4248 opened by @ashleysommer several months ago, with the formatting fixes and a new test. I take zero credit for the original fix!

Proposed fixes:

From the original PR:

Some authentication plugins implement chained auth functions on actions such as user_create and user_update. If no other plugins provide these functions, then CKAN raises the exception "The auth 'user_create' is not found for chained auth". The CKAN built-in auth system however does provide these functions. So it makes sense to fallback to having the chained auth function chain to the corresponding built-in function.

This is an example of one such plugin which attempts to chain off 'user_create' and 'user_update' and fails:
https://github.com/NaturalHistoryMuseum/ckanext-ldap/blob/6f9aec4edadaf792e347b4d20adad2c2322c167f/ckanext/ldap/logic/auth/create.py#L12

I also fixed one other small bug in authz.py: the error message displays the auth_function name, but chained functions are partial functions and don't have names, so it was failing. It now uses the name of the .func if it's a partial.

Features:

• includes tests covering changes
• includes updated documentation
• includes user-visible changes
• includes API changes
• includes bugfix for possible backport

Please [X] all the boxes above that apply

[amercader]

@wardi I took the liberty of reviewing, fixing and merging this as I was working on it.