Conversation view blocks sending to left groups

Instead of attempting to send, which will throw an 'Unknown group' exception,
show a toast.

// FREEBIE

_locales/en/messages.json

@@ -1,4 +1,8 @@
 {
+    "youLeftTheGroup": {
+      "message": "You left the group",
+      "description": "Displayed when a user can't send a message because they have left the group"
+    },
     "debugLogExplanation": {
       "message": "This log will be posted publicly online for contributors to view. You may examine and edit it before submitting."
     },

js/views/conversation_view.js

@@ -15,6 +15,11 @@
             return { toastMessage: i18n('unblockToSend') };
         }
     });
+    Whisper.LeftGroupToast = Whisper.ToastView.extend({
+        render_attributes: function() {
+            return { toastMessage: i18n('youLeftTheGroup') };
+        }
+    });
 
     var MenuView = Whisper.View.extend({
         toggleMenu: function() {
@@ -343,12 +348,15 @@
             var toast;
             if (extension.expired()) {
                 toast = new Whisper.ExpiredToast();
-                toast.$el.insertAfter(this.$el);
-                toast.render();
-                return;
             }
             if (this.model.isPrivate() && storage.isBlocked(this.model.id)) {
                 toast = new Whisper.BlockedToast();
+            }
+            if (!this.model.isPrivate() && this.model.get('left')) {
+                toast = new Whisper.LeftGroupToast();
+            }
+
+            if (toast) {
                 toast.$el.insertAfter(this.$el);
                 toast.render();
                 return;