Add veterans from waitlist to flight

Adds a specified number of veterans from the waitlist to the flight.

The selection follows these rules:

• Veterans are taken in order of application date
• When a veteran in a group is selected, all group members are added
• Veterans with paired guardians have their guardian added automatically
• Flight history entries are recorded for each assignment

Each veteran is saved, then each paired guardian. A CouchDB 409 is retried once after the current revision is re-read. If that retry still conflicts, the failure is a conflict for that document.

The response is 200 only when every attempted save succeeds. When a veteran or guardian save fails, the status is 409 if any remaining failure is a conflict and 500 otherwise. Both failure responses list the ids that were saved and the ids that failed.

The veteranCount must be between 1 and 100.