Enhance lesson management in GitHub Sync plugin

- Added support for lesson pages: created, updated, and removed counts.
- Implemented processing of lesson directories and their YAML files.
- Updated the database schema to include an 'itemid' field in the mapping table.
- Introduced new language strings for lesson page operations.
- Updated versioning to 0.7.0 and adjusted upgrade scripts for database changes.

Author: allanhaggett