Got back to hacking today, and did something I've wanted to do for some
time. Made all the assistant's threads be managed by a thread manager. This
allows restarting threads if they crash, by clicking a button in the
webapp. It also will allow for other features later, like stopping and
starting the watcher thread, to pause the assistant adding local files.

[[!img /assistant/crashrecovery.png]]

I added the haskell async library as a dependency, which made this pretty
easy to implement. The only hitch is that async's documentation is not
clear about how it handles asyncronous exceptions. It took me quite a while
to work out why the errors I'd inserted into threads to test were crashing
the whole program rather than being caught!
