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

Can remove the "add handouts" part of the `TODO` statement on L85.