[taylortom]

Fixes #55

Fix

• schema/course.schema.json: declare the course globals at _globals._components._youtube, where the template and core read them, instead of _globals._youtube
• migrations/v3.js: add a migration that moves an existing flat _globals._youtube into _globals._components._youtube, merging with any nested values (the flat values win, as they are the ones that were edited), then removes the flat key. Courses without a flat _globals._youtube are left unchanged

Testing

1. Copy the plugin into a framework's src/components and run grunt migration:test --file=adapt-youtube; all adapt-youtube tests pass, including the new ones (flat only; flat plus nested; nested only; no globals; plugin not installed)
2. Run the migration on a course whose course.json has _globals._youtube.skipToTranscript set: the value moves to _globals._components._youtube.skipToTranscript and _globals._youtube is removed
3. Validate a course with _globals._components._youtube values against the updated schema: the values are kept