Content Store: Use db:prepare instead of db:setup

We probably don't want to nuke existing databases anyway, and the new
SQL-instead-of-Ruby based schema in Content Store won't apply cleanly to
an existing database (where this will just make it run migrations
instead).

projects/content-store/Makefile

@@ -1,2 +1,2 @@
 content-store: bundle-content-store router-api publishing-api
-	$(GOVUK_DOCKER) run $@-lite bin/rake db:setup
+	$(GOVUK_DOCKER) run $@-lite bin/rake db:prepare