Further caching fixes

[Oaphi]

This PR ensures fixes caching behavior for user notifications (currently, in production we allow those to get cached for a day with no regard to new notifications). It also makes it possible to test response caching in development as it turns out Rack::MiniProfiler overwrites cache control headers even if perform_caching is set to true. Finally, it also fixes a DoubleRenderError when requesting thread_content twice in a row (that's not in production yet, I made a mistake when adding fresh_when with respond_to).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.82%. Comparing base (e2a295b) to head (369d071).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	71.56% <100.00%> (+0.04%)	⬆️
helpers	82.01% <ø> (ø)
jobs	60.00% <ø> (ø)
models	89.02% <ø> (ø)
tasks	61.11% <ø> (ø)

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[cellio]

Tested and notifications showed up promptly in my dev environment, for what that's worth. I see that Art has already reviewed the code, so LGTM.