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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[cellio]

I set up a Stripe sandbox. Donations above the minimum go through (correct UI for us, shows up in Stripe, sends email). Donations below the minimum are caught in the form so they're never submitted. I don't yet know what we do with webhooks or how to test that.

[cellio]

I haven't been able to trigger this message; an amount below 0.5 gets intercepted in the view with a message that just says "can't be below 0.5" without the explanation. This PR doesn't change that behavior, but I just wanted to note that in case it's testing error on my part and I should be able to make this show up.