Migrations integration

[taylortom]

Work to integrate migrations mechanism from the framework into the authoring tool.

• Migrations should run:
  • On AAT upgrade - this is currently handled by at-utils, speak to @taylortom before doing this bit
  • On plugin update
  • On course import
• No interactive UI element (users will simply be notified of a success/error)
• No write protection to be included in initial pass - with the expectation that we'll use feature being developed by @chris-steele