Add new container page that can display nested xblocks

This change introduces a new container page which will eventually replace the current unit page for [STUD-1244](https://edx-wiki.atlassian.net/browse/STUD-1244). This new page is able to display nested xblocks, and in particular can show nested child verticals as collapsible sections inline.

In addition, the unit page has been updated to link all child containers over to the new container page. This makes it possible for the user to view the details of these nested xblocks. Note that the new page is currently only read-only, and a subsequent story will add editing support.

The design can be found on the wiki: [Provide a new read-only view for container XBlocks](https://edx-wiki.atlassian.net/wiki/display/OPENPROD/Provide+a+new+read-only+view+for+container+XBlocks)