Fix positioning of paw sprites

Fixes #2815. 

* Adjusted position of paw sprites using absolute positioning instead of margin-top. 
* Added a class of `pets-rare` to the Rare Pets menu in `stable.jade` in order to target the link around the golden paw sprite.
* Adjusted position of Flying Pig sprites to leave more room between row above. (Don't think they need to look as if they're flying in this view and when you have a whole row of pigs, the gap between rows looks odd).

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User: Pixel