XBlocks can disable navigation chrome.

@nedbat @cpennington There is an option to:
* Enable/disable accordion navigation chrome
* Enable/disable/repoint tab navigation chrome

This allows for full-screen XBlocks (e.g. a code IDE, or large video
player). It is also the first pass at allowing top-level XBlocks. It's
also now possible to make a chromeless XBlock, point a tab to it, and
make it point back to that tab.

Next steps down that path would be:
* Fix up how tabs are handled. The current version is a hack.
* Create appropriate XBlocks for courseware, tabbed navigation,
  etc. to reach feature parity
* Invert/rejigger the XML format.
I found some weird code in this template. Before merging the PR, we will want to fix this up. The 'accordion' variable was being repurposed here for reasons I don't fully understand. 

I suspect the current version breaks verified certs (so please do not merge), but I wanted to open a PR in part to figure out what we should do here.