Allow Arcane Shield to overwrite existing shield

-Will replace an existing damage shield entirely if both the shield
value and duration would be greater on the new shield.
-Will update the current shield value (but not max value) of an
existing shield if the new value would be greater. We keep the old
max value because the player may want to buff their shield with Aegis.
-If we updated an existing shield, we will refresh the duration to
3 turns unless the duration has already been refresh 20 times. This
checks the same property used by Weapon of Light and Bathe in Light,
which is why I allowed refreshing 20 times instead of a smaller amount.
-It's unlikely that an existing shield would be refreshed many times by
Arcane Shield, but I implemented this limitation to avoid refreshing
the same shield indefinitely. T think Aegis is the only talent that
benefits from refreshing an existing shield versus creating a new one,
so there shouldn't be any real abuse potential here.
-Note that this will never update or replace an existing shield if the
current duration is greater than Arcane Shield's default duration. We
can't know whether the player values shield value or duration more, and
updating the shield value of an existing damage shield with greater
duration would introduce power creep.