Better Mop and Cart love

Slight tweak to the behavior of attacking the janicart with a mop.
Changed it so that it stores the mop when it is soaked (or the bucket is empty), as I'm sure most players would prefer their mop to always be fully soaked when they remove it from the cart.

Also changed the examine text from 'water' to 'liquid' as it was misleading. It can actually be filled with any reagent, even welder fuel...or acid... or blood... or dr.gibb...space lube... you get the picture.
changed wetness check from constant to variable reference
Merge branch 'master' of https://github.com/tgstation/-tg-station into bucketlove
Merge pull request #586 from Malkevin/bucketlove

Better Mop and Cart love