[DO NOT MERGE] Initial flat representation of Container View in Studio.

The following pull request is being used to gather initial feedback and discussion around some of the architectural SASS, pattern library, and HTML template implications of the Container View story. 

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 patterns we would like to set for a number of other projects. 

@talbs, @frrrances  - I've opened up this pull request to better track FED (front end development) feedback and improvements. I'm also happy to walk through in person an initial explanation of the work to date, though I hope to update documentation soon that should also help. The plan was to further discuss UI/UX aspects of this story in review meetings next week, since there are still open questions on that front. 

@andy-armstrong, @singingwolfboy - I've also opened this pull request to start the conversation about how to potentially move the container.html page into the platform as a reference html file (which would involve some changes not currently reflected in the branch.) I'd previously mentioned to Andy that I could separate out styling from the HTML of this pull request, but it may make sense to update it to land the container.html page as a reference item only visible on development environments. 

How to test this locally: To test this, open up any unit page in Studio and change the word "unit" in the URL to "container." This will show you the flat, demo version of the container page.

I will update this pull request soon with updated documentation to include a better representation of the FED work done. You can view current documentation here: https://edx-wiki.atlassian.net/wiki/display/OPENPROD/Provide+a+new+read-only+view+for+container+XBlocks