XBlocks can disable navigation chrome.

There is an option to:
* Enable/disable accordion navigation
* Enable/disable/repoint tab navigation

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.
Removed suppress_top_level_nav, as per Cale
Explicit disable_accordion
Review from Cale
Review from Cale
Merge pull request #4174 from edx/pmitros/chromeless-xblocks

XBlocks can disable navigation chrome.

This allows for full-screen XBlocks (such as code IDEs), as well as allowing us to somewhat hackishly build top-level XBlocks. This is the first step on the path to non-hackish top-level XBlocks.
Explicitly skip tests in common unless we know we're running the LMS unittest suite