The config forms containers had a rule to remove the top/bottom margins

for the first/last .djblets-l-config-forms-container element within a

parent. This worked for the usage in RBCommons, where we wanted some

components (like lists) to appear toward the top/bottom of a box without

adding additional margins. This was done with :first-child and

:last-child rules, which didn't work unless the layout was just

right.
This change removes those rules and replaces them with new modifiers for

making the container flush against the top or bottom, which just removes

any margins/borders. This is easier to reason about and tweak in

consuming applications.

Tested this with some container usage for integrations, and played with

styles in the browser.