Set auto create user

[frankiejol]

Currently, users are auto created when granted access from external auth. As we foresee new authentication methods as pointed by @fv3rdugo in #2050, we need to add a setting to enable or disable the auto creation of users.
When disabled, and administrator must upload users and groups using the recent batch creation methods added in v2.2.

[fv3rdugo]

Yes, with #2050 we will have Authentication. We have to consider the authorization and already the accounting (AAA) more information about https://en.wikipedia.org/wiki/AAA_(computer_security)

[frankiejol]

As for the accounting we already log every user request, though there are no reports implemented yet. Information can be inspected right from the database.