management: Add new create_realm management command.
management: Add new create_realm management command.

This is intended for rare situations where one is creating multiple
realms via a script.

After all the preparatory refactoring in this last several commits, we
can now provide a working implementation of a create_realm management
command.

andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,kou/zulip,kou/zulip,kou/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,kou/zulip,andersk/zulip,kou/zulip,kou/zulip,kou/zulip,zulip/zulip