refactor(sprites): create individual sprites from mounts spritesheet

Previously, Mounts lived on a separate spritesheet from all other site graphics, and the CSS for their individual elements was manually generated. This commit doesn't change that setup yet, but creates the individual image files needed to compile mount sprites into the site master spritesheet using grunt-spritesmith. (The task won't run on my machine, so someone else needs to execute that step!)
refactor(sprites): grunt spritesmith for @sabrecat's e44c84bef906be836a23d30bcbad3fa148f4be20
refactor(sprites): remove deprecated Mount-SpriteSheet.png and strip down Mounts.css

This commit removes the old mounts spritesheet, and removes all manually created mount CSS from Mounts.css, leaving only the 18px top margin used to position mounts in the herobox.
Merge pull request #109 from HabitRPG/sabe/tweak

move mounts to spritesmith system