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

To make sure the conversion of the relative URL is done by `course-updates-list` and not by the modulestore, you can add a quick check here with a call to `get_item` to make sure it still includes the `/static`.