[PLT-4318] Display a message when post is over 4000 characters (#4687)

* test changes

* added functionality

* css updates

* additional css updates

* i18n updates

* textbox cleanup

* var naming tweak

* replaced jQuery with React-based UI changes

webapp/sass/responsive/_tablet.scss

@@ -27,6 +27,12 @@
 
             .control-label {
                 top: 0;
+
+                &.post-error {
+                  left: 32px;
+                  top: 0px;
+                  position: relative;
+                }
             }
         }