Add PDF export functionality for knowledge posts

[matthewwardrop]

This PR addresses #298 by adding support for exporting knowledge posts to PDFs using weasyprint, as well as exposing support for this format in the web app user interface. At this point, the code works, but there are no stylesheets are applied, and so it is very much a work in progress.

Auto-reviewers: @NiharikaRay @matthewwardrop @earthmancash @danfrankj

[NiharikaRay]

Thanks for doing this!

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✅ Build knowledge-repo 1.0.748 completed (commit bbf4f399af by @bulam)

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✅ Build knowledge-repo 1.0.750 completed (commit 1f62844d2c by @bulam)

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✅ Build knowledge-repo 1.0.751 completed (commit 2529e3d005 by @bulam)

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✅ Build knowledge-repo 1.0.754 completed (commit 1b27f84883 by @bulam)

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✅ Build knowledge-repo 1.0.756 completed (commit c587463bc0 by @bulam)

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❌ Build knowledge-repo 1.0.758 failed (commit dfc6b2ae7b by @bulam)

[bulam]

I made the following changes:

• Rewrite the if statement so that the PDF download function works along with the other download options
• Remove the post format argument since the resource type argument was sufficient
• Change PDF default font to Helvetica to match the KR default font more closely
• Add weasyprint as a dependency. Limited to older versions of weasyprint to prevent build failures due to Python 2 incompatibility

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✅ Build knowledge-repo 1.0.760 completed (commit 63fc510a15 by @bulam)

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✅ Build knowledge-repo 1.0.762 completed (commit 168e0bbfc1 by @bulam)

[bulam]

It's been a long time since this PR was updated and it looks like a lot of improvements to weasyprint have been made in the meantime? Maybe you can bump the pinned version.

Edit: oh, I saw the point about Python 2 compatibility. Just open another PR dropping Python 2 from the CI ;)

Ha yea maybe for the next PR :P

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✅ Build knowledge-repo 1.0.764 completed (commit f3a3f1acbc by @bulam)