MDL-46950 gradereport_grader: Consolidate styling of the elements

The grader table has been made entirely standalone from core or
theme styling. The floating divs have to share the exact same
rules, and so it was a lot easier and more maintainable to just
style them both using the same selectors, entirely ignoring the
table default styles.

And because the selectors are unique for both the floating elements
and the table cells, a themer can easily re-use them to style both,
without having to dig around to find all the selectors to overwrite.

Working on this highlighted some issues with the positioning. Those
have been fixed within the same patch. And even though it is still
not 100% correct on IE, the result is pretty decent.
MDL-46950 gradereport_grader: Prettier styling for base and clean