Fix undefined secondary sort for user posts breaking pagination

[Oaphi]

closes #2008

This PR adds a secondary order by creation timestamp to user's posts (newest first) - this should be safe enough for 99.99% cases (creation timestamp collision is highly unlikely). In addition, the PR lays the groundwork for a proper action callback by making the logic for setting posts in the show and posts actions shared (that's as far as I am willing to go in scope for this particular fix).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.74%. Comparing base (4deef91) to head (7314863).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	75.86% <100.00%> (+<0.01%)	⬆️
helpers	85.06% <ø> (ø)
jobs	66.88% <ø> (ø)
models	93.01% <ø> (ø)
tasks	61.11% <ø> (ø)

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[cellio]

This changed the number in the "see all" button on the profile from the true number back to 15 (for someone with more than 15, i.e. someone with pagination).

[Oaphi]

This changed the number in the "see all" button on the profile from the true number back to 15 (for someone with more than 15, i.e. someone with pagination).

ah, whoops, a sec

[cellio]

LGTM now; tested and it's behaving correctly. Thanks for adding the test.