Change response format of /api/pleroma/emoji so it makes sense

When tags were introduced the returned format changed from { shortcode1: "url", shortcode2: "url", shortcode3: "url" } to [{ shortcode1: {...} },{ shortcode2: {...} }, { shortcode3: {...} }] which does not make any sense and is harder to parse. This MR changes it to { shortcode1: {..}, shortcode2: {...}, shortcode3: {...} }