Add fallback for modal overlay in IE8.

[DFurnes]

Changes

• Fixes modal background in IE8. The fallback PNG was removed when we split modals into a separate package. Applying the fix here for now, since I don't want to have to duplicate the fallback PNG across multiple repos.

For review: @DoSomething/front-end

[DFurnes]

I think my preference is to keep all of our styles in Neue, and just have dosomething-modal hold modal logic (showing, hiding, transitions, etc) with a lightweight "starter" style that's more general-purpose in the modal package.

But that's something we can think more on & decide later on.

[weerd]

Makes sense. It should be more specific to the project that implements the modal, that decides what the background would otherwise be and appropriate fallback :) :+1: