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/responsive/_mobile.scss

@@ -1073,6 +1073,11 @@
                     margin: .5em 0;
                     top: 0;
                 }
+
+                .post-error {
+                    top: 0;
+                    left: 0;
+              }
             }
         }
 

webapp/sass/responsive/_tablet.scss

@@ -42,6 +42,10 @@
                 top: 0;
                 left: 32px;
                 position: relative;
+
+                .sidebar--right & {
+                    left: 0;
+                }
             }
         }