This function takes the stream name (a string) and using numbers
makes it seem like it's actually using a stream id, which is
confusing. This change adjusts `stream` to always be a string.
This lets us simplify the long-ish ‘../../static/js’ paths, and will
remove the need for the ‘zrequire’ wrapper.
Signed-off-by: Anders Kaseorg <anders@zulip.com>