In the old config forms, spacing between elements and within the parent

were partly defined by the stylesheet and partly left up to the

consumer. This meant that some uses had to be specially tweaked to get

elements to look correct. Some of the built-in styles made this more

difficult, with unwanted borders and margins in places.
This change defines a new .djblets-l-config-forms-container class,

which is intended to be used whenever there's a form element, text

description, set of buttons, or anything else that's displayed in a

config form. This defines a standard 1em margin between other containers

and between the parent (which is now recommended to provide no padding).
It also allows the contents within to be recessed into the parent

container, replacing the old .box-recessed. This is useful for

components like lists.

Made use of this with some new UI backed by config forms.
Tested that all existing config forms UIs we have were unaffected by the

style changes, meaning the backwards compatibility worked.