Fix for accidental insertions of "[]()" into fields upon hitting Enter in various post fields + general cleanup

[Oaphi]

closes #1058

The PR also:

• enables TypeScript checks for our community assets (and fixes remaining type-related errors in them);
• moves remaining markdown-related helpers to our QPixel.MD common API global object;
• fixes runtime error when using the judasim.js community asset without the sidebar notice setting filled in;
• fixes Sefaria Linker clobbering post source in edit mode (it links every occurrence of the source text by default);

Since the PR touches upon community assets, DO NOT MERGE until both @trichoplax and @cellio approve (that is, unless you check that those assets work fine with changes yourself).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.56%. Comparing base (578ef87) to head (0caa26e).
⚠️ Report is 14 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	74.66% <ø> (ø)
helpers	84.91% <ø> (ø)
jobs	79.24% <ø> (ø)
models	90.01% <ø> (ø)
tasks	61.11% <ø> (ø)

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[trichoplax]

What I understand looks good to me, but I don't have an understanding of TypeScript yet so I'll leave it to someone else to approve those changes.

So this is just to confirm that I've tested that the code golf assets still work and that pressing Enter in the Edit Comment field while editing a post no longer adds an empty Markdown link to the Body field. Pressing Enter in the Edit Comment field does now save the edit, which I assume is intentional (seems reasonable behaviour to me).

[cellio]

I've tested the Sefaria linker with the latest changes and all looks good now. I assume that other reviewers did a code review and tested Code Golf.