Draft: Add a shield effect and make blood barrier give a shield
Adds a shield effect that mitigates a flat amount of incoming damage. On the client a purple shield bar is displayed and updates when the shield takes damage.
Not sure where to display shield life yet, and right now the BB shield is pretty weak compared to the heal since it expires after the duration of the frenzy. Thoughts are in #1672
Also includes fixes for syncExtend to others (syncExtend syncs to all, new syncExtendSelf method) and syncExtend from effect init.