Update: Read/write themeVariables from config instead of course

[taylortom]

Update

• Theme editor now reads themeVariables from Origin.editor.data.config instead of Origin.editor.data.course
• Theme editor now saves themeVariables to the config model instead of the course model

Testing

1. Start with npm run dev from adapt-authoring/
2. Open theme editor — verify theme variables load correctly from config
3. Modify theme variables and save — verify changes are saved to config record in MongoDB
4. Verify no theme data remains on the course record after saving

Fixes #449
Depends on adapt-security/adapt-authoring-coursetheme#55
Related framework PR: adaptlearning/adapt_framework#3578