Bug 986604: add webmaker video

Good news everyone! We actually already have the Less for fully fluid videos in webmaker.org.
<https://github.com/mozilla/webmaker.org/blob/master/less/explore/competency.less#L111-L130>
You can see it in action at <https://github.com/mozilla/webmaker.org/blob/master/public/views/competency.html#L87-L91> (that outer div should probably be a figure. i'll open a bug.)

The outer element is optional, but adds a grain of control over the embedded object.

I'm going to open a bug to add fluid videos makerstrap. I'll be changing the classnames to
```less
  .video-container {
    max-width: 100%;
    width: 60rem;
  }
  .video-letterbox {
    height: 0;
    overflow: hidden;
    padding: 3rem 0 56.25%;
    position: relative;

    iframe, object, embed {
      left: 0;
      height: 100%;
      position: absolute;
      top: 0;
      width: 100%;
    }
  }
```