Add EXTRA_XBLOCK_SETTINGS setting, send to XBlocks

* Adds platform configuration variable EXTRA_XBLOCK_SETTINGS, which gets
  passed to the instantiation of XBlocks as "platform_settings".

* XBlocks then exposes these "platform_settings" to XBlock objects via
  self.runtime.platform_settings
This is the real change; these other modified import lines are just reordering to be more sensible.