Implement Auto-AFK (#192)

- Channel-configurable delay
- User is marked AFK if no chat messages are received before the delay expires
- User is marked un-AFK if a chat message is received or if the user voteskips

www/channeloptions.html

@@ -56,6 +56,13 @@
                 <input type="text" id="opt_maxlength" placeholder="HH:MM:SS">
             </div>
         </div>
+        <!-- auto afk -->
+        <div class="control-group">
+            <label class="control-label" for="opt_afktimeout">Auto AFK Delay (0 to disable)</label>
+            <div class="controls">
+                <input type="text" id="opt_afktimeout" placeholder="0">
+            </div>
+        </div>
         <hr>
         <strong>Admin-Only Controls</strong>
         <!-- page title -->