`rake db:create` creates a schema when `schema_search_path` is specified

Closes #3624

After this patch `rake db:create` will create a schema if `schema_search_path` is specified. Currently it only works if `schema_search_path` is a single schema. I've seen a couple of tickets related to `schema_search_path` so I first wanted to ask what format we actually support. If something like `first, second` is a valid `schema_search_path` of course I'd have to adjust the patch to create two schemas.