Add missing key-terms slug to DefinedTerm URL tag

[Scotchester]

Our contacts at Google alerted us to an oversight of mine when I implemented our DefinedTerm markup: the URL for each term that goes in the <link itemprop="url" href="{URL}"> tag was missing the key-terms/ part of the URL. Turns out, our use of RoutablePageMixin to generate the Key Terms pages meant that the page.full_url tag was not including that key-terms/ part of the URL.

I tried using the routablepageurl template tag, but that only returns a root-relative URL, whereas we need an absolute URL, and it required hardcoding a category='key-terms' kwarg, anyway, so we might as well just hardcode it this way.

/cc @kelleyholden

How to test this PR

1. Visit http://localhost:8000/consumer-tools/auto-loans/answers/key-terms/, inspect the markup for a term, and see that the <link itemprop="url" …> tag's href attribute does not include the key-terms/ path segment
2. Pull branch
3. Reload the page and see that the key-terms/ is now included

Checklist

• PR has an informative and human-readable title
  • PR titles are used to generate the change log in releases; good ones make that easier to scan.
  • Consider prefixing, e.g., "Mega Menu: fix layout bug", or "Docs: Update Docker installation instructions".
• Changes are limited to a single goal (no scope creep)
• Code follows the standards laid out in the CFPB development guidelines

[Scotchester]

Thanks for the assist, @higs4281! Updated it to insert the slug in the term URL.