Get dashboard by slug or UUID

╭⁽˙͡˙̮ ⁾╮ Gives us nicer urls like `/edit/name-of-dashboard` ╭⁽˙͡˙̮ ⁾╮
(੭•̀ᴗ•̀)੭ Backwards compatibility with `/edit/391213f0-336f-11e4-8c21-0800200c9a66` (੭•̀ᴗ•̀)੭