Fix a 500 error when previewing CertificatePage from CMS

[rachellougee]

What are the relevant tickets?

NA
I noticed it while working on the edx course certificate backfill, as I needed to view an older certificate revision

Description (What does it do?)

Fixes a 500 error when previewing a CertificatePage revision. The learner's certificate page is unaffected.

• e.g., https://mitxonline.mit.edu/cms/pages/68/revisions/2585/view/ accessed from https://mitxonline.mit.edu/cms/pages/68/history/

How can this be tested?

• Make sure the course page and certificate page are setup in http://mitxonline.odl.local:8013/cms
• Make some changes to the course certificate page

- The preview should now work as expected

Additional Context

[github-actions]

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