Adds a "open in popup" button to channels.
Opens a conversation in a new compact window.
Requires a modern browser (ES2015 support).
Since it simply opens a new Discord instance and hides the unnecessary clutter in it, you can safely close the original window (unlike the way popups work on Google Hangouts for example).
You can also force hiding everything except the chat by adding ?chat_only at the end of the url and reloading the page.
