First rough cut of a container page.
Couple more changes
updated container header styling and exposed sidebar back to its location. includes some javascript to render tooltips on overflowed text headers
initial static representation of a basic xblock rendering structure in container.scss, not a finished product but useful as a base commit to cherry pick into the production implementation of these xblocks on a container page, which will be build dynamically and recursively within a page.
added a temporary collapsed class to show how you might have a container view variant for an xblock header achieved only through css. may need more discovery to think of a more general way to handle this. does not include the has-actions button yet, will tackle that challenge next.
modification of the sass pipeline to separate out general xblock styling from the container view, as well as modifications to the container view to move closer towards a read only container view made specific to ab experiments
updated visual styling and class naming for container view, added in html structure to the header area to support both a details section on the left and an action button area on the right.
incorporated interaction hover styling and transitions, added styling for header-actions even though these will be hidden for the read-only version of the container page to start. removed unused and refactored styling from _container.scss that was initially based on _unit.scss, though much of the editing, drag-drop, and adding of new components remains as it was on the unit page.
added view button styling for xblock-type-container
updated navigational header for reference container page to better reflect launch navigational workflow needs for this page. also updated styling for element headers, removing the page level header and changing the styling of the nesting and element level headers
Add support for UX reference template that can only be seen in developer mode.

These can be viewed at: /template/ux/reference/index.html

Also checked in two initial style references which are linked from the index:
  container.html - a potential draft for the new generic Container page
  unit.html - the current Unit page
Remove accidental change
revert of styling changes and container page changes that somehow disappeared through a git rebase operation.
further cleanup of missing rebase styling, addressing styling PR comments and feedback
cleaned up xblocks to not use line-height, use flexbox support, in addition to other pull request review feedback.
removed extra unit page styling from container, ensured proper container styling on unit page, addressed styling reuse for action lists, cleaned up todo comments
changed nesting structure of xblocks and container styling to improve sass readability and minimize obtuse property overwritting
removed commented out older styling for xblocks and containers
now using flex-grid for pae structure instead of relying on older unit page structure
moved from mixin to extend in xblock styling and addressed font sizing issues