Increase resistance threshold from 1 to 50

Hide Magic EH bars for those magic schools for which you have equipped at
least 50 resistance.  Previously, any level greater than zero resulted in
the bar being displayed.
Cap physical mitigation at 75% for EH calculation.

This wasn't capped, meaning armor past the maximum mitigation was still
considered.  This was most visible when targeting very low monsters.

Fixes #12