Downloads page for multiple files

As originally suggested in #3175606: [PLAN] 8.x-1.0 Release (over 3 years ago!), some people might like to have "the ability to have a downloads page vs BinaryFileResponse if multiple files are added to the form".

That said, I don't personally see much need for this, especially now that the handler plugin has been built to allow multiple instances, and tokens can specify each individual handler instance. So I don't expect to implement this any time soon, unless anyone comments here asking for it and/or is up for contributing the solution!