Make the grinder more useful by balancing repair bonuses

Currently the grinder is useless for repairs, because the same result can be achieved with crafting. Crafting repairs should be disabled.

Another fix is to balance out repair bonuses. If not disabled, repairing with crafting should give a total durability that is less than two items combined. A grinder should add like 10%. The anvil adds 20% and keep enchantments.