Fixed #12103 -- Added AuthenticationForm.confirm_login_allowed to allow customizing the logic policy.

Thanks ejucovy and lasko for work on the patch.
Fixed #20649 -- Allowed blank field display to be defined in the initial list of choices.
Removed unused model option "admin"
Fixed #20667 - Removed discussion of DEBUG from tutorial.

Forward-port of 3493f18d78 from master.
Fixed #17519 -- Fixed missing SQL constraints to proxy models.

Thanks thibaultj for the report, jenh for the patch,
and charettes for the tests.
Fixed #18777 -- Localized form fields with as_text/as_hidden

Thanks croldan for the report.
Fixed #20838 -- Fixed Geodjango spatialrefsys test failure with postgis-2.0.3
Fixed #20786 -- Cleaned up docs/ref/exceptions.txt

Thanks Daniele Procida for the suggestion and edits.
Fixed #18681 -- GenericInlineModelAdmin.get_formset() no longer bypass get_fieldsets().

Refs 23e1b59 which already fixed this issue for ModelAdmin and InlineModelAdmin.
Introduced ModelAdmin.get_fields() and refactored get_fieldsets() to use it.

Refs #18681.