Initial styling for Container View in Studio along with UX reference template area.

This pull request is for the initial styling for the Container View, including the sass separation of generic xblock wrapper and header styling, a ux templates area for testing visible only in development mode, as well as static representation of the HTML used in the dynamically generated Container View.

@andy-armstrong has a follow on pull request that builds the dynamic HTML templates for the container page using the work from this pull request. 

The impetus of this is to work towards read-only support for A/B content experiments in Studio, though as I mention above it also represents a great opportunity to think (over the next couple sprints) of some general FED patterns we would like to set for a number of other projects. 

Check out these changes and then view:
http://localhost:8001/template/ux/reference/container.html
http://localhost:8001/template/ux/reference/unit.html