Add markdown tools to the ban message textarea

Now we can ban in style!
Perhaps something could be made to fetch things from [reddit's Crowdin page](https://crowdin.com/project/reddit)?

Also, thanks for accepting this! I realize though that I made a small mistake on like `1404`, it should be this instead:

    		return ['you have been banned from posting to [/r/' + subreddit + ': ' + title + '](/r/' + subreddit + ').',

Would you like me to make another PR for this? or is it something small enough you'd work it in later?