Revert "get rid of mob.see_in_dark and species.darksight"

This reverts commit 278a4c38ec.

code/modules/spells/spellbook.dm

@@ -252,6 +252,7 @@
 							if (!(XRAY in H.mutations))
 								H.mutations.Add(XRAY)
 								H.sight |= (SEE_MOBS|SEE_OBJS|SEE_TURFS)
+								H.see_in_dark = 8
 								H.see_invisible = SEE_INVISIBLE_LEVEL_TWO
 								H << "\blue The walls suddenly disappear."
 							temp = "You have purchased a scrying orb, and gained x-ray vision."