Fix comment thread expansion if its title contains clickable elements

[Oaphi]

closes #2036

Also makes sure we do not replace the inline thread with the fetched content unless the response indicates success (most likely solves meta:295715, but let's make sure every case is covered in a separate PR).

As a treat, it also ensures MathJax stays properly rendered upon collapsing previously opened comment threads.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.88%. Comparing base (7b6dfd1) to head (78b5da0).
⚠️ Report is 8 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	76.18% <ø> (ø)
helpers	85.09% <ø> (ø)
jobs	66.88% <ø> (ø)
models	93.04% <ø> (ø)
tasks	61.11% <ø> (ø)

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[Oaphi]

Let's do a production deployment upon review & merge

[cellio]

Tested and works for me.

Before the fix, in a dev environment I got what looks like a stack trace rather than a 404, so I'm not sure how to test the "maybe also fixes" issue. But we can check that case after deploying this and see if it's still an issue.

[Oaphi]

Before the fix, in a dev environment I got what looks like a stack trace rather than a 404

That's the exact same error, @cellio, it's just that in development mode the ErrorsController is not invoked by default (for the dev to be able to see the trace and other info). In production, the same error results in the 404 error page being served.