Improve adding artwork form: required fields + error feedback

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.

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

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