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.
Update main.json
feat(quests): allow quests to drop multiple items
Merge pull request #109 from HabitRPG/sabe/tweak

move mounts to spritesmith system
chore(grunt): grunt with @sabrecat's mount=>spritesmith, + my multi-drops
feat(quests): add the gryphon quest, mount, pet, egg. For the glory of @baconsaur!
fix(quests): don't drop canBuy:false items
chore(css): mounts consolidation
fix(locales): Remove spurious nbsp from English locale settings page
Update main.json
Merge pull request #107 from luveluen/develop

Update main.json
improve(translations): add translations for achievement modals
Revert "improve(translations): add translations for achievement modals"

This reverts commit c7f07e3522c9cb5ea76b2a1509cd668e8442eaef.
Add strings for views/shared/modals folder
Add missing strings
improve(translation): merge @Sinza- pr
Test quests valid values
fix(translations): Remove more spurious NBSPs from English translation