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.
Some files already were using `noop` in place of `() => {}`.
It's both clearer what it means and is easier to type.
This updates all test files to fully use `noop`, and
adds a shared import from the test lib file.
This replaces one of the our larger remaining blocks of code with
explicit logic for message_lists.home with logic that should correct
if we support maintaining a different set of rendered message lists.
This should have identical behavior to the previous implementation
given that all_rendered_message_lists is either just
message_lists.home or that plus the current narrowed message list;
can_apply_locally() is always true for the home message list, and the
list === message_lists.current check is equivalent to the more complex
narrow_state.active() check.
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>