Feature: Server-sent Events (SSE) for dance lists.

This involves Uvicorn and Django-Channels for async support. Visiting the list edit page for a dance list subscribes you to an event channel which distributes additions and changes to the list. This means that the “Add to list” button on a dance detail page not just adds the dance to the currently-active list, but the new dance is also displayed on the editing page immediately without a reload. It also means that group-visible lists can be edited by all members of the group, even concurrently, although of course there are operations which can conflict.