Add dropdown select for # issue tag
    
    Since the library we are currently using does not support multiple
    delimiters in the same text area and has a very inactive dev,
    change to use At.js.
    
    Example1 (issue): http://i.imgur.com/yZYGamA.png
    Example2 (user): http://i.imgur.com/jzh3SFa.png
    
    * Use @ to reference a user
    * Use # to reference an issue/pr
    * NOTE: when using markdown formatting for headers, the dropdown will
    still appear. This is consistent with behavior in github and others
    though