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

According to the documentation for updates, it returns plain text non-html strings  see https://github.com/edx/edx-platform/blob/master/lms/djangoapps/mobile_api/course_info/views.py#L31