Island visitor protection from monsters

[Muspah]

Commit de0ce64 (#1057) added a (default off) configuration option to protect island visitors from damage from monsters on the island. The implementation actually just checked on the EntityDamageEvent if getEntity() was an instanceof Monster if the protection was enabled. That caused issue #1099 without actually protecting the visitor.

PR #1128 sort-of fixed this by checking the DamageCause, but needs some rework to cancel potion-effects applied by witches instead of just cancelling damage from magic/poison.