This adds some early design tokens for constructing data tables. A data

table is meant to be a reusable, accessible component for managing

tabular data, and eventually a new base for such components like

Djblets's datagrids or Review Board's issue summary table. However, it's

a bigger task, and likely out of scope for our immediate deadlines.
The design tokens are still useful, though, so those have been split out

so that other components providing their own tables can make use of

them.

Made use of these in Review Board and Djblets for tables.