xblock-settings: Add service to allow access to settings from XBlocks (WIP)

Work in progress, initial version to allow access to settings from XBlocks.

To do:
* Figure out why we need to add the service in the ModuleStore object init - adding it to the LMSModuleSystem should be enough, but from the Ooyala XBlock `student_view()`, only the i18n service seem to be listed as a service.
* Better define the way to set settings meant for XBlocks - maybe a specific dict instead of the whole LMS settings environment?
* Add tests