move automerge to webworker

Currently automerge blocks the main browser event loop which leads to a feeling of transcribee "hanging" while it loads documents. We should avoid that by moving automerge to a webworker

Start by locating where Automerge is initialized and where documents load in the browser, then identify the webworker entry point or worker setup. Done means Automerge work no longer blocks the main event loop while documents load and the existing loading behavior still works.

Written by the indexing model from the issue text.