Historically, Djblets's datagrid styles have been quite ugly and

somewhat broken. We've generally handled styling within Review Board

itself, placing styles there partly for convenience and partly to match

Review Board's look and feel. However, with Ink, we're now in a better

position to improve the default styling.
This change moves the styling to Ink, backports a lot of the Review

Board styles to Djblets, and introduces dark mode.
The styling is based on the (in-progress) Ink data table component. In

the future, the plan is to rebuild datagrids on top of a more stable

Ink foundation, but that's out of scope for now.

Tested with Review Board, with an in-progress change that removes much

of the overridden styling in favor of Djblets.
This was tested on the dashboard, All Review Requests, Users, Groups,

and the user page.