MDL-46567 gradereport_history: Initial commit with basic structure of the report
MDL-46567 gradereport_history: Prevent duplicate username column from rendering
MDL-46567 gradereport_history: Fix bug with paging
MDL-46567 gradereport_history: Move paging implementation to db layer
MDL-46567 gradereport_history: Extra version.php info
MDL-46567 gradereport_history: Basic readme.md
MDL-46567 gradereport_history: Basic codestyle, phpdocs and other minor cleanups

This commit includes following changes:-
- Fix codestyle issues
- Remove readme file
- Fix and add file headers
- Fix and add missing phpdocs