Minor improvements to close reasons handling

[Oaphi]

closes #1717

Also cleans up the controller a little bit and documents instance variables / variables expected by the /close_reasons/form view just in case

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.72%. Comparing base (78c1f6e) to head (e9f19f2).
⚠️ Report is 8 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	69.06% <100.00%> (+0.10%)	⬆️
helpers	79.01% <ø> (ø)
jobs	48.57% <ø> (ø)
models	85.90% <ø> (ø)

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