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.
tabs,accordion does not equal None. The lists are explicit overrides. If we add a new piece of chrome to the edX platform, the behavior ought to be: 

None ==> Shows all three pieces of chrome
tabs,accordion ==> Explicitly shows just those two pieces of chrome

Likewise, if we were to kill the accordion in the default layout, we might have tabs,accordion bring it back. None goes to the default. 

This is especially important for how legacy courses are handled.