Fixes beastmaster to not count gryphons

Splits up drop and quest pets, and only counts the former (on the stable screen, and for beastmaster achievement). Mount screen now doesn't include rare mounts in its count. Also added additional unit tests for pet count, mount count, and beastmaster achievement.

This was done by creating content.dropEggs and content.questEggs, in addition to the original content.eggs. If anyone has a cute way of combining dropEggs and questEggs into content.eggs automatically, that would be extra-lovely; at the moment, the lists are written out manually, and will all have to be updated with new pets (assuming a more elegant solution isn't implemented by the time the next pet quest is released). Note that there is probably excess code in both content.dropEggs and content.questEggs, but I wasn't sure what was safe to remove.

This should resolve issue #2676 (https://github.com/HabitRPG/habitrpg/issues/2676), by separating quest pets both visually and achievement-wise. Note: please double-check that I didn't screw up spacing between lines of pets in stable.jade; my local install is showing wide spacing between each row, and I can't find any obvious cause.

(See pull request #2801 on habitrpg for rest of code)