Move themeVariables storage from course to config

[taylortom]

Summary

The Adapt framework (PR adaptlearning/adapt_framework#3578) adds support for reading themeVariables from config.json during grunt/LESS compilation, generating both LESS variables and CSS custom properties. Currently the AAT stores these in course.themeVariables, but the framework expects them in config.json.

Changes needed

• writeCustomLess(): Read themeVariables from config data first, fall back to course data for unmigrated courses
• processFileVariables(): Accept variables parameter instead of reading from course.data.themeVariables
• applyHandler() / removeHandler(): Find config record by _courseId + _type: 'config' and update it instead of the course record
• processCustomStyling(): Accept variables parameter directly
• Version-based LESS generation: Use semver to check framework version; skip LESS variable generation when framework handles it natively
• Import migration: Register content migration to move course.themeVariables → config.themeVariables on course import
• Add semver dependency

Backward compatibility

• Build fallback: configData[attributeKey] || courseData[attributeKey] || {} handles unmigrated courses
• Import migration handles old exports with themeVariables on course