Add error to RHS reply box for messages > 4000 chars, consistent with create post and edit post errors (#4871)

* functionality

* CSS updates

* cleanup

* moved message length checks to Textbox component

* cleanup

webapp/sass/layout/_sidebar-right.scss

@@ -59,6 +59,24 @@
         form {
             padding: .5em 15px 0;
         }
+
+        .post-create-footer {
+            @include clearfix;
+            font-size: 13px;
+            overflow: visible;
+            position: relative;
+            clear: both;
+
+            .post-error {
+                font-weight: normal;
+                margin-bottom: 0;
+                display: inline-block;
+                font-size: .85em;
+                @include opacity(.55);
+                position: absolute;
+                top: -25px;
+          }
+       }
     }
 
     .help__format-text {