Allow stations to orbit star in system they are in

Backend and frontend updates allowing station's movement strategy
to be able to be set to an elliptical instance.

If set the UI will pickup the movement and render the orbit
line as well
Use updated omega API in bin/util/omega-missions-retrieve
Support subsystem wide events in missions subsystem

Expand mission event heirarchy to support specific
mission related event types (victory, expired, etc) and
add missions::subscribe_to and missions::unsubscribe
json-rpc methods

Now user can subscribe to generic mission completion
events, filtering out the events which they are notified
about by mission id, assigned to user id, and other metadata

Specs have been updated though I suspect have been broken
by recent changes (hopefully will have time to go in and
update these soon)