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

We generally verify `response.status_code` before accessing `response.content`.   (This way, if there's ever a real issue in the future, it will be easier to debug since we'll know that the status_code is not as expected.)