Simplify post followed by user method

[trichoplax]

Separate out a small self contained change from #1548 as practice at using git cherry-pick.

Move a method from the CommentThread model to the Post model to make its usage more readable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.75%. Comparing base (6d955a4) to head (55760b9).
⚠️ Report is 10 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	70.86% <100.00%> (+0.28%)	⬆️
helpers	81.92% <ø> (ø)
jobs	60.00% <ø> (ø)
models	86.87% <100.00%> (ø)
tasks	61.11% <ø> (ø)

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