Fix attribution information for CC0

[Aspie96]

Fixes

Fixes #207 by @panchovm

Description

This issue fixes the attribution for CC0 correcting it from "is licensed under" to "is marked with".

Technical details

Patch #221 was accepted for the same issue, but what it does is actually incorrect, as is the explaination in #207 (comment).

That patch does not actually change the attribution generated for any license nor for CC0. What it does is changing the attribution in the footer (which is of a CC BY 4.0 work) from "licensed under a" to "marked with a", which makes little sense since CC BY is a license.

I fixed it back to "licensed", however writing "licensed under the" instead of "licensed under a". The reasoning is that, while the first is actually common, there is only one CC BY International 4.0 license, so I think "the" makes more sense. Either way "marked as" should never have been in the footer, since that was for the generated attribution for CC0 works.

Tests

Testing this PR is quite easy.

I generated a few examples of attributions, both for CC0 and for licenses, and it seems to work, both for the preview and the generated HTML.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[obulat]

Sorry for late review, I've been working on the second version changes to the Chooser. Thank you for working on this and your attention to details, @Aspie96 !
I refactored some of the files, and it's been a little tricky integrating your changes: I've merged the master branch into your branch, and added a small change to the Recommended license card.