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.
If you make this default to `tabs,accordion`, rather than `None`, then you should be able to just code against the expectation that it's always a string.