Allow consentform.html file URL to be passed in

[flatslats]

Being able to localize the consent form is very important, and yet there is no easy way to do it. Please add a consent form file URL property so we can pass in the URL to the localized asset we want to use (similar to how the privacy policy URL is passed in), rather than hardcoding "consentform.html".

This would make localization trivial on both iOS and Android (with the unfortunate duplication of the HTML files, but they're pretty small and few in number).