Use High-DPI emote images when available

[noodlebox]

Use srcset to specify 2x- and 4x-scale versions of emote images when available. Twitch emotes always have larger versions available, scaling up smaller emotes when necessary. Larger versions of FFZ emotes return 404 when not available, so checking the API data first will be necessary.

A less elegant alternative would be to attempt loading the largest size first for any emote, falling back to the default size on error.