Navigation sidebar sorting

[rebeccaalpert]

@rohitkrai03 reports that Developer Lightspeed would like to have an ability to sort conversations in the navigation sidebar. Happy to contribute as well.

We want to make the search input part of a toolbar with custom actions passed in by the consumer. They need the divider line as well.

[rebeccaalpert]

Closing in favor of #768 so product folks are aware of work.