zproject: Add prod_settings mypy stub, aliasing prod_settings_template.
zproject: Add prod_settings mypy stub, aliasing prod_settings_template.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,rht/zulip,zulip/zulip,kou/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip