Balance tweaks to purge sectoid (buff/nerf)

Purge Commandos and Purge Mecs can be advent leaders for the assassinate the advent general mission type. This can only occur in the early Force levels though.

If using UFOpedia the Purge unit has Biographies now.

Tweaks to the purge sniper's detonation Shot ability. Tweaks to the Cybernetics and Fire resist perks coding. Done by Rusty Dios.

Purge Mecs no longer has Blast Padding and have a new form of Kinetic Plating.