CSS: add `overflow: hidden;` to chat messages

Stops messages being able to obscure other messages with weird unicode
characters.

anonstream/static/style.css

@@ -188,6 +188,7 @@ noscript {
   width: 100%;
   box-sizing: border-box;
   border-radius: 4px;
+  overflow: hidden;
 }
 .chat-message:hover {
   background-color: #434347;

anonstream/templates/nojs_chat_messages.html

@@ -98,6 +98,7 @@
       box-sizing: border-box;
       border-radius: 4px;
       transform: rotate(-180deg);
+      overflow: hidden;
     }
     .chat-message:hover {
       background-color: #434347;