Supply controller now holds a ref to it's shuttle

Instead of having to look it up every time.
shuttle_controller now uses a shuttle datum
Fix for supply controller update
Adds escape pods to the controller
Forgot to process shuttles
Some fixes
Minor shuttle fixes and cleanup

Readds the missing "shuttle has left the station" message, improves the
ETA calculation and reorganizes the procs in shuttle_controller.dm
Merge pull request #5330 from mwerezak/emergency-shuttle

Updates emergency shuttle controller to use shuttle datums