uploads: allow uploading multiple files from your computer at once.

Allow uploading multiple files from your computer at once.

The maximum limit is 100 files at once. There is still a 50MB size limit
that applies to the whole upload. This limit is at the Nginx level.

The upload widget no longer shows a thumbnail preview of the uploaded
file. This is because there isn't room for it in a multi-file upload,
and because the next page will show a preview anyway after the files are
uploaded.

Direct file uploads are processed synchronously, so they may be slow.

API change: the `POST /uploads` endpoint now expects the param to be
`upload[files][]`, not `upload[file]`.

app/components/file_upload_component/file_upload_component.scss

@@ -31,4 +31,8 @@
       background-color: var(--uploads-dropzone-progress-bar-foreground-color);
     }
   }
+
+  &.dz-error .dz-error-message {
+    display: block;
+  }
 }