The recent change to spacing units left data table cell and header sizes

a bit too big for comfort. Both the horizontal and vertical sizes were a

step too big, making them appear huge and blocky.
This brings them down to the sizing units reflecting the sizes they were

before.
Along with the fix, the horizontal and vertical sizing have been split

into new CSS variables, so that other styles can better reflect the

correct sizes.

Checked Review Board's datagrids, Commits table, and change fields.
Fixes for those will also require changes in Djblets and Review Board.