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 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 commit renames 'settings_muted_topics.js'
to 'settings_user_topics.js' because the file now
supports the settings for topics with any visibility_policy,
not just MUTED.
It also renames the corresponding test file.
Evy Kassirer
Prakhar Pratyush
Anders Kaseorg
Lalit