Fixed #21435 -- Improved error message when urlconf is empty.

The new error message now hints that the most likely issue
is a circular import.

Thanks to trac user elena for the report and to
bpeschier for the original patch.
Fixed #22123 -- EmailValidator now also accepts IPv6 literals in addresses
Fixed #21902 -- Documented search order for list_display.
Fixed #22120 -- Documented persistent activation of languages and cleaned up language session key use
Fixed #22092 -- Documented ResolverMatch.view_name
Extra new line to make flake8 happy