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

I understand the desire to create small-scoped unit test methods for each case.  However, `test_handout_static_rewrites` is just a superset of `test_handout_exists` and adds to the duplication (and forward maintenance) of test code.