Fix issue where illegal transitions were possible from EBS
Closes #42 (closed)
This MR introduces the following changes:
- Requesting EBS will not reset the AMI state to AMI_NOT_SELECTED, so that new missions cannot be selected from EBS.
- EBS will not be available when AMI state is AMI_NOT_SELECTED (This is to prevent the above behavior from happening at all).
- EBS requests will be rejected when EBS is already activated.
Note that the only theoretically valid AS_STATE with AMI_NOT_SELECTED is AS_OFF (after this change). Since the rules do not specify EBS behavior for AS_OFF, we can disable it.