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/components/create_post.jsx

@@ -297,6 +297,9 @@ export default class CreatePost extends React.Component {
         const fileInfos = Object.assign([], this.state.fileInfos);
         const uploadsInProgress = this.state.uploadsInProgress;
 
+        // Clear previous errors
+        this.handleUploadError(null);
+
         // id can either be the id of an uploaded file or the client id of an in progress upload
         let index = fileInfos.findIndex((info) => info.id === id);
         if (index === -1) {