Hide resize handle for both private and group chats

Fixes #1154

css/converse.css

@@ -7487,7 +7487,8 @@ body.reset {
       width: 100%;
       border: none;
       min-height: 60px;
-      margin-bottom: -4px; }
+      margin-bottom: -4px;
+      resize: none; }
       #conversejs .chatbox .sendXMPPMessage .chat-textarea.spoiler {
         height: 42px; }
       #conversejs .chatbox .sendXMPPMessage .chat-textarea.correcting {

sass/_chatbox.scss

@@ -248,6 +248,7 @@
                 border: none;
                 min-height: $chat-textarea-height;
                 margin-bottom: -4px; // Not clear why this is necessar :(
+                resize: none;
                 &.spoiler {
                     height: 42px;
                 }