This is a prep commit for typescript migration of this module. In this commit
I made all the users of ListWidget pass generic sort functions like 'alphabetic'
and 'numeric' explicitly to avoid type conversions which we may need when moving
to typescript, also it simplifies our `set_sorting_function` a bit more.
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>