Style the green 'online now' dot entirely in CSS.

This lets us clean up the HTML a little bit in preparation
for a later change which will cause the stream and people
lists to scroll independently of one another.

Also it feels a bit more fun.

(imported from commit b3b49149d7ec2960fd752fe50b41e55d363c1a98)

zephyr/static/js/ui.js

@@ -924,7 +924,7 @@ exports.set_presence_list = function(users, presence_info) {
                                               e.preventDefault();
                                           });
         if (presence_info[email]) {
-            user.prepend($('<img>').addClass('active-icon').attr('src', '/static/images/green-dot.png'));
+            user.addClass('active-icon');
         }
 
         $('#user_presences').append(user);