Use modernized classes for Djblets config forms list items and actions.

Review Request #11009 — Created May 6, 2020 and submitted — Latest diff uploaded




We've introduced component classes for the Djblets config forms, but
this wasn't used everywhere. The lists, list items, and actions were
still using old-style classes, which wasn't very future-proof and made
it a bit hard to mix in with more modern codebases. There was an idea
before to keep supporting legacy classes here for another release, but
in the end it just wasn't worth it, and we've moved beyond that

This change updates the list items to use the newer classes, and to
formalize classes for list item actions (removing the reliance on
overloading .btn and stuff). There's a few tiny style tweaks coming
along for the ride as well, to help reduce unwanted whitespace around
lists of actions.

The new definition for item actions also includes a new -is-primary
modifier, and switches the existing danger and disabled to
-is-danger and -is-disabled. These are not specially styled, as we
don't want to conflict with a consuming application's own styling. It's
up to them to offer their own button styling.

Tested with some of our existing config forms pages, and with the
upcoming extensions rework.