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

elsewhere.
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.