Improve /emoji

[Sv443]

• Make the buttons auto-download the emoji

  If the MIME type is application/octet-stream and the URL path ends in .png, the browsers should automatically download it
  However this means we need to download the image to our server and then serve it under the different mime type or figure out how to pipe through the emoji data from the CDN while modifying the request

• Parse emoji URLs in the emoji argument, so that emojis from servers people haven't joined can also be downloaded
• Maybe support automatically retrieving the emojis from replying to a message with a /emoji reply cmd
• Add a context command for retrieving a message's emojis (Add context command equivalent of /emoji #124)