Migrate chat view to view model

The view model listens to events to update the chat view on the fly when new messages arrive or are sent. Also adds the ability to actually send messages. In the second commit, all the Briar API calls were finally moved out of Composables by slightly rewriting the sidebar logic that was still relying on it (adding yet another view model).

Closes #95 (closed), #69 (closed), #90 (closed)

This PR builds on top of !33 (merged) and should be merged after it. Currently the base branch is set to !33 (merged) for an easier diff.