Multiplayer focused shield enchantment: Legion

Legion: When shields sharing the same banner design are in X range, the block rating increases +1, up to +2 with lv1, +3 with lv2, and +5 with lv3

There's no interesting shield enchantments, and absolutely none that are multiplayer-specific. This fixes both problems in one fell swoop. For reference, if I'm not mistaken, the vanilla shield rating is a 5, so it'd half knockback, more or less, at lv3, with a group of 6 players.