Added the new item "Cleric Ring"

A new item "Cleric Ring" has been added. If a unit has it, the amount of fatigue added when using the staff is halved (rounded up to the nearest whole number). This item cannot be traded on the map.

This effect was implemented as an answer to a comment that the fatigue from using staves is too severe. I have changed the too large numbers so that they only apply on hard and higher difficulties, but I wanted to make sure that even in hard mode, you can find some fun in managing with these items.

Of course, this item is also available in normal mode, making it easier to use staves in normal mode without worrying about fatigue.

Items that cannot be traded on the map cannot be stolen. To take something that an enemy has, you must use the Thief staff.