Should just return one gateway code.
Get rid of wgDonationInterfaceAllowedHtmlForms and selection weight.
Logic should be about like this:
- Get list of all enabled gateways
- Read configurations for all of them
- Compare country, currency, payment_method from query string against config to narrow down list of gateways
- Use tiebreaker function created in T302935 to just return one gateway
Replace contents of DonationInterfaceFormSettings.php with config for tiebreaker function that makes the selected gateways consistent with original logic (enforced by unit tests created in T302891)