feat: pdf block enabled by default

[Kelketek]

Description

This PR enables the PDF block by default, per the product proposal where it was to be added.

Testing instructions

1. Create a new course.
2. Don't modify the advanced modules for the course.
3. Create a unit and check the advanced dropdown. Add a PDF block for the course and verify it works as expected.

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[kdmccormick]

Presuming you're able to validate it locally or on the sandbox, LGTM!

[Kelketek]

Thanks, @kdmccormick . It is definitely working for me locally. Sandbox seems to be choking on deployment but it seems unrelated to me. I don't have merge access, so feel free to merge when the tests pass if you're happy.