This updates our datagrids to remove a bunch of CSS overrides that are

now present in Djblets, and to convert what's left on the pages to dark

mode.
Styling updates include the content header, sidebar, custom columns, and

user page.
The user page has had a tweak to the tabs. We now use rb-c-tabs

instead of a custom tab implementation, ensuring we use the same style

consistently. This is primarily a styling update. In the future, this

will move to an Ink component, once that's written.
There's also a fix for the Groups page, which was using the default

template and not our custom template. This was made apparent with the

removal of some of our custom styling.

Unit tests pass.
Tested all the datagrid pages in light and dark mode.
See https://reviews.reviewboard.org/r/13809/ for screenshots.