Gives the STAB flag to rapiers, fixes STAB weapons damage info.

Also makes it so that weapons with the STAB flag are displayed as having piercing damage on the info screen, same as SPEAR flag weapons.

https://github.com/CleverRaven/Cataclysm-DDA/issues/4527 & https://github.com/CleverRaven/Cataclysm-DDA/pull/4525