It doesn't make sense for us to track a separate current filter when
it should just the be filter of current message list if there is one.
This will reduce possible confusion in the codebase where filter
returned by narrow_state is different from message_lists.current.
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>