Manager now sends out task updates via SocketIO, and the web interface
handles those.
Note that there is a `BroadcastTaskUpdate()` function, but not a
`BroadcastNewTask`. The 'new job' broadcast is sent after the job's
tasks have been created, and thus there is no need for a separate
broadcast per task.
SocketIO clients can now send a message with `/subscription` event type
in order to subscribe to or unsubscribe from job-related updates.
These job-related updates themselves aren't sent yet, so this is a change
that's impossible to really test. The socketIO code for joining/leaving
rooms is called, though.
