initial_password: Add explicit development environment assertion.
initial_password: Add explicit development environment assertion.

The construction of INITIAL_PASSWORD_SALT is such that it should only
be set in development environments, but we should enforce this rule.

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