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.. although the doc may imply that, here's what the API returns:
http://mobile3.m.sandbox.edx.org/api/mobile/v0.5/course_info/MITx/6.002x_4x/3T2014/updates

```
    {
        "date": "June 21, 2014", 
        "content": "<section class=\"update-description\"><p><b>Register for 6.002x to earn a certificate!</b></p>\n  <ul>\n  <p>A new run of 6.002x will begin on August 25, 2014! Visit the <a href=\"https://www.edx.org/course/mitx/mitx-6-002x-circuits-electronics-2606#.U6Y_7I0bBgu\">course About page on edX</a> to learn more and register to earn a certificate of achievement.<p>\n   <p>In the meantime, we hope you will continue to explore, learn, and participate in the discussions. We will email and post an update again before the next run of the course begins. <p>\n    <p>-- The 6.002x course team </p>\n          ", 
        "status": "visible", 
        "id": 19
    }
```