Block encoded thumbnail URLs ('data:image')

These only work in the `selectThumbnail` component. If it's small enough, it'll get past the transaction limit-check, but ultimately it doesn't appear correctly in the claim list.

It'll probably appear correctly if we tweak the ClaimPreview code, but since this also fails the Google Video metadata test, I think we should disallow this.

## Issue
5923

ui/component/selectThumbnail/view.jsx

@@ -35,7 +35,7 @@ class SelectThumbnail extends React.PureComponent<Props> {
 
     updatePublishForm({
       thumbnail: newThumbnail,
-      thumbnailError: false,
+      thumbnailError: newThumbnail.startsWith('data:image'),
     });
   }