PLT-4943: Warn when editing a message > 4000 chars (#4789)

* added basic functionality

* css fixes

* css fixes part II: electric boogaloo

* removed css interference with theme changes, for this and post length error

webapp/sass/responsive/_tablet.scss

@@ -32,14 +32,10 @@
                 display: none;
             }
 
-            .control-label {
+            .post-error {
                 top: 0;
-
-                &.post-error {
-                    left: 32px;
-                    position: relative;
-                    top: 0;
-                }
+                left: 32px;
+                position: relative;
             }
         }