auth: Automate calculation of get_auth_backends_data.
auth: Automate calculation of get_auth_backends_data.

This change lets us eliminate the need for new authentication backends
to edit get_auth_backends_data, since we're just computing it from the
official registry in zproject/backends.py.  Should save a few lines of
work whenever we add a new auth backend, and make that more accessible
to new contributors.

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