Make `[+]` invisible when there's nothing to be expanded

[felixmulder]

If a member doesn't have a docstring, it shouldn't have a [+]. Nor should it have one if the expanded docstring is equal to the shortened.

Implementation details

• In entities.scala, add a field to entity def hasShortenedComment
• In JavaConverters.scala, add this field to converted entities
• In api-page.css, add a css rule to turn the [+] the same color as the background (so we can keep the same spacing)
• In the api-page.html, add the appropriate fixes to set the above css rule when applicable

[felixmulder]

Fixed in #2023