Update Modal to be responsive

[benfurber]

We're planning to encourage visitors to leave an (optional) donation whenever they download a resource file.

This would be best be done on page in a modal. Our modal currently uses fixed widths and heights for the small actions they're currently used for which won't work for the more complex use this would involve.

So this issue is about enabling that more responsively sized component/children.

[benfurber]

I was going to prepare this before adding the donation stage, but I realise I'm needing to add context to the modal to test my changes, so I think adding the donation component first is simplier.