1090 fix

[benlk]

Changes

• allows the top term to be a term in the post-type taxonomy
• change the id for the "None" top term option from string 'null' to string 'none', because it might've been undocumented type conversion.
• internal documentation of the function largo_top_term
• rework of the logic of largo_top_term, closing an edge case where a post with the following characteristics would not have a top term link output:
  • category set
  • no top term explicitly set
  • created in the database instead of through the post editor, because that's the only way that such a combo would exist, likely

Why

Because the @!@#$%^&* top term wasn't showing up.

For RNS-104.

[benlk]

Please let's write tests for this eventually.