Anton/sync groups and verticals

This fixes https://openedx.atlassian.net/browse/BLD-1095

NB: before merge `ENABLE_GROUP_CONFIGURATIONS` should be set to default value.

First phase reviews: @tymofij  @olmar 
This is due to the way xmodules are handled by the LMS now. Calling `get_module` just sets up the descriptor to act as an XModule, rather than an XModuleDescriptor (this is how we bridge the gap between XModules which have 2 classes, and XBlocks which only have 1)