RFE: add option to auto-mothball units on long transits

[SuperStucco]

Units can be mothballed to avoid maintenance checks and costs, which can be valuable when making long transits.

A campaign setting to auto-initiate mothballing by unit type in this circumstance would be welcome. This would be a number indicating the maximum number of jumps without mothballing units, with a check box for each unit type (checked for auto-mothball, unchecked for ignore). When transit to another system is initiated past that number of jumps, the specified unit types would start to be mothballed by the designated techs. When the formation jumps to the destination system the units are brought out of mothball while the DropShips are in transit.