MDL-35716 report_performance: Performance report for moodle site

This will report config setting which can affect moodle performance.
Few of them are added in this patch:
1. Theme designer mode
2. Cache Javascript
3. Debug message set to developer
4. Backup - Active
5. Stats enabled.
MDL-35716 report_performance: Changed doc links for issues
MDL-35716 report_performance: Integrated changes suggested by Sam M
MDL-35716 report_performance: code refractored to have classes