MDL-40347 Grade report does not highlight some cells properly

- added class 'i123' to each cell in the column
- changed JS to highlight the cells with this class instead that cells with the column number
- changed grade report css so the .vmarked and .hmarked classes have higher priority for all rows
- made cells in 'average range' row td instead of th
- little corrections to css: removed background image for some cells with .header css class
- MyMobile theme disabled cells highlighting, make sure it disables it in all cases