Organizing the mounts page the same way the pets page has been for a while

So this isn't the best code in the world because it's mostly been trial and error, and I felt like I was fumbling around in the dark the whole time. But even if it's not perfect I think it brings a lot more structure to the mounts page than we've had previously. I'm making a pull request in habitrpg-shared as well, to give content.coffee and index.coffee their own mount updates as well.

Commits 2 and 3 here are the same file, with the difference being whether there are paw print icons underneath the Rare Mounts heading. In commit 2 (no paw prints) there are arrows, and in commit 3 there doesn't seem to be a hover balloon with the mount's name, so I think I messed something up. I'll try to fiddle with them some more tomorrow.



and, erm, my UUID is 1534a0ee-2f69-4c2e-9659-0d719ac2ea68 :3