remove NONE as a license option for SWORD, show available licenses #8551

[pdurbin]

What this PR does / why we need it:

The example XML in the SWORD docs doesn't work due to licenses changes in 5.10 and PR #7920.

Which issue(s) this PR closes:

• Closes As of 5.10, SWORD API no longer accepts NONE for dcterms:license #8551

Special notes for your reviewer:

I added some minimal testing of interactions between license and rights. We could add more.

I'm not 100% sure of all the assertions in the SWORD page of the API Guide. We could add more tests and/or edit the guide as needed.

Generally speaking, I'm thinking we should drop NONE as an option, even if it's an incompatible change. If we want to keep NONE, what should it map to? Custom Dataset Terms? But they can't be blank, right?

Suggestions on how to test this:

• Try passing NONE. You should now see a list of possible licenses.
• Try passing a valid license.
• Test all the assertions in the SWORD section of the API Guide about license, rights, etc.

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

No.

Is there a release notes update needed for this change?:

Maybe?

Additional documentation:

None.

[qqmyers]

Looks OK - I didn't run it.

If I understand, the only main code change is to add the list of active licenses in the response.
My comment there suggests adding a check to make sure the request License is active, which would be a minor functionality change.

The other changes fix the example, add good tests, update docs. I saw one more CC0 that could be changed but it all makes sense.

[coveralls]

Coverage decreased (-0.001%) to 18.896% when pulling 09e1c60 on 8551-sword-license into e4eb675 on develop.