Improve adding artwork form: required fields + error feedback

[jvolker]

The adding artwork form could be improved.

• Required fields should be marked accordingly.
• Submitting without a URL or artwork format selected shows an empty red box without any error message:
  It should mention the missing required fields.
• Submitting without a title for the artwork shows:
  The `Artwork` instance is not valid. Details: `title` can't be blank (value: undefined).
  This could be shortened to `Title` can't be blank

Tested with latest master.

[jvolker]

Also, I think adding a thumbnail URL should be required for public artworks to keep the public stream visually appealing.