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

Hmm.. It may be a bigger change at this point, but I would suggest the following:

Change `static_replace` so that all callers pass in an explicit `replacement_function`, using the design pattern you introduced.  The various replacement_functions would be defined in that file itself so it's clear what we support.  For example:
1. build_absolute_url(request, course) (used by mobile_api)
2. build_from_data_directory(data_directory)
3. build_for_course(course_id)

Right now, it's a mix of multiple design patterns (optional parameters that dictate how to replace, plus a replacement function).

Alternatively, keep the current design pattern and add an optional `request` parameter that would be used to build an absolute URL.

Or is there another alternative?