Migration

Added a brief list of what's likely to be needed.

The current approach we're taking with this for Opigno is to work on an in-site migration for only Opigno entity types to LMS, leaving the rest of the site as-is. e.g. install LMS + ecosystem modules on a site, migrate data to the new entity types, uninstall Opigno modules. There should be some source/destination plugins and/or migration templates available for this soonish, not sure exactly in what format.