create-db.sql: Start by dropping the zulip database if needed.

At some point the PostgreSQL Docker image started creating the zulip
database for us, which caused our CREATE DATABASE to fail.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

scripts/setup/create-db.sql

@@ -1,3 +1,5 @@
+\connect postgres
+DROP DATABASE IF EXISTS zulip;
 DO $$BEGIN
     CREATE USER zulip;
 EXCEPTION WHEN duplicate_object THEN