Update mirror angle in GUI while it's moving

[alexdewar]

Since #743, we've been able to get the position of the mirror, even while the motor's moving. However, the GUI just displays "moving" for the mirror angle in this case. It would be better to display the actual angle instead, so that the user knows that something is happening even if they can't see it (e.g. if they're on an aircraft). It probably makes sense to do this at half-second intervals or something.

To do this, we will need a new pair of pubsub messages for querying the mirror angle and sending the response.