Make static URLs absolute in handouts and about for mobile API

Same as https://github.com/edx/edx-platform/pull/6116 but for the release candidate instead of master.

@gwprice @nasthagiri just review this one and I'll make sure everything gets updated for the master one.

cc @BenjiLee this is what we want to get into the release

This helper does almost nothing interesting without reference to a specific request, and bringing requests into that file didn't feel right, since right now it's all just functions on strings. I agree it doesn't really belong in views. Do you have any suggestions for a better place? I'm also willing to be talked into putting it in static_replace if that's really the best place for it.