Allow multiple units in a crate

... (closes #2165)

By specifying multiple units in a crate, a squad can be given to the player, for example:

    GiveUnitCrateAction@squad:
        SelectionShares: 20
        Unit: e1,e1,e3,e3

The first unit specified is used for determining whether the crate can be given to the user, so for example: 

        Unit: e1,e1,e3,e3

will be used for both sides (soviet and allies), while

        Unit: e2,e1,e3,e3

can be used to allow this type of crate for soviets only