General improvements for donations handling

[Oaphi]

• :intent with :amount less than 0.5 no longer causes a runtime error due to a typo in a local variable name (@symbol);
• return_to parameter is now dropped unless it's an HTTP, HTTP(S), or relative URI (security);
• :intent and :success have tests now;

If you want to test donations manually, you'll only need an API key for a sandbox.

Do not merge unless at least one requested reviewer manually confirms that the full flow works.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.84%. Comparing base (612ac9b) to head (2295bc2).

Files with missing lines	Patch %	Lines
app/controllers/donations_controller.rb	64.70%	12 Missing ⚠️

Additional details and impacted files

Components	Coverage Δ
controllers	77.80% <64.70%> (+0.77%)	⬆️
helpers	85.58% <100.00%> (+0.10%)	⬆️
jobs	79.93% <ø> (ø)
models	93.48% <ø> (ø)
tasks	61.11% <ø> (ø)
scripts	∅ <ø> (∅)

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