restored the GasGiant supply malus that was accidentally removed in r7795, and refactored the colony size of 3 as a subsitution macro MIN_RECOLONIZING_SIZE

git-svn-id: svn://svn.code.sf.net/p/freeorion/code/trunk@7796 dbd5520b-6a0a-0410-a553-f54834df5b05