Add ChartPlugin and metadata for nvd3 and BigNumber vis (#6085)

* add nvd3 thumbnails

* add ChartPlugin and metadata files

* add BigNumber thumbnails

* Add bignumber ChartPlugin

superset/assets/src/visualizations/BigNumberTotal/BigNumberTotalChartPlugin.js

@@ -0,0 +1,20 @@
+import ChartPlugin from '../core/models/ChartPlugin';
+import ChartMetadata from '../core/models/ChartMetadata';
+import transformProps from '../BigNumber/transformProps';
+import thumbnail from './images/thumbnail.png';
+
+const metadata = new ChartMetadata({
+  name: 'Big Number',
+  description: '',
+  thumbnail,
+});
+
+export default class BigNumberTotalChartPlugin extends ChartPlugin {
+  constructor() {
+    super({
+      metadata,
+      transformProps,
+      loadChart: () => import('../BigNumber/BigNumber.jsx'),
+    });
+  }
+}