JSON response handling improvements

[Oaphi]

This PR further improves our handling of responses to JSON requests:

• adds a QPixel#parseJSONResponse helper preventing silent failures when our API responds unexpectedly (such as 422 Unprocessable entity with text/html type);
• adds several helper methods for standard handling of AJAX actions (vote, retractVote);
• fixes a couple of inconsistencies with what we return in the status field;
• makes several client API methods response parsing-safe & adds last resort notifications for end users;

I don't have time to apply these fixes everywhere at once, but I tried to cover as many cases as I can right now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.44%. Comparing base (16d0278) to head (3453a96).
⚠️ Report is 22 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	71.26% <100.00%> (+0.38%)	⬆️
helpers	81.96% <ø> (ø)
jobs	60.00% <ø> (ø)
models	88.36% <ø> (ø)
tasks	61.11% <ø> (ø)

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