Thumbnail editing UI looks broken

[thibaudcolas]

In the audio/video edit views, the thumbnail editing UI doesn't look right:

• "Change thumbnail file: Currently:" provides the same info as the thumbnail's file name + link above it
• "Clear" and its checkbox aren't side by side
• "Change" isn't styled like "Clear" is

Tested with Wagtail 2.3.

This seems like it might have been caused by changes in Wagtail's styles that this extension might be relying on, but I didn't investigate further.

[thenewguy]

This is because the template duplicates the widget output.

Should probably leave this to the widget and remove the anchor rendered by the template.

Could probably update the file widget to be the same here and simplify in one issue