PLT-137: Disable upload button when max uploads reached (#5053)

* Change upload button color opacity when max reached

* Display max upload message

* Remove error when deleting preview

* Clear error message in side-bar

when user reaches max upload and error is displayed in side-bar, removing one file won't remove error message

* Scroll in preview after file upload in sidebar

webapp/sass/layout/_post.scss

@@ -437,6 +437,15 @@
                 @include opacity(.9);
                 box-shadow: none;
             }
+
+            &.btn-file__disabled {
+                @include opacity(.1);
+                
+                &:hover,
+                &:active {
+                    @include opacity(.1);
+                }
+            }
         }
 
         textarea {