auth: Fix error on startup in django-two-factor-auth in Django 2.1+.
auth: Fix error on startup in django-two-factor-auth in Django 2.1+.

https://github.com/Bouke/django-two-factor-auth/issues/297
This setting was added in 1.9 version of the app and can be used
harmleslly in our current Django 1.11-based code and will prevent an
error on Django 2.1+ when we move there.

punchagan/zulip,kou/zulip,timabbott/zulip,brainwane/zulip,zulip/zulip,eeshangarg/zulip,showell/zulip,kou/zulip,synicalsyntax/zulip,timabbott/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,zulip/zulip,shubhamdhama/zulip,punchagan/zulip,showell/zulip,andersk/zulip,brainwane/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,shubhamdhama/zulip,synicalsyntax/zulip,rht/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,shubhamdhama/zulip,kou/zulip,synicalsyntax/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,timabbott/zulip,punchagan/zulip,showell/zulip,shubhamdhama/zulip,shubhamdhama/zulip,brainwane/zulip,eeshangarg/zulip,brainwane/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,brainwane/zulip,showell/zulip,timabbott/zulip,kou/zulip,zulip/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,synicalsyntax/zulip,hackerkid/zulip,showell/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,brainwane/zulip,kou/zulip,shubhamdhama/zulip,timabbott/zulip,timabbott/zulip,hackerkid/zulip,shubhamdhama/zulip,hackerkid/zulip,kou/zulip,showell/zulip,kou/zulip,andersk/zulip,rht/zulip,synicalsyntax/zulip,synicalsyntax/zulip,andersk/zulip,timabbott/zulip,hackerkid/zulip,showell/zulip