The config forms styles in Djblets were somewhat incomplete, leaving

some important formatting (such as field errors and help text) to the

consuming applications. It also just didn't look very good by default in

some areas, such as the text fields, and required too much internal

knowledge in order to adjust the available widths of the forms.
This change backports a lot of the styling from RBCommons, which is a

heavier user of the config forms:

Fixes the styling for error displays and radio button lists
Fixes up lots of spacing and sizing issues for buttons, labels,

  paragraphs, and other parts of the UI
Adds a clean, rounded border for text fields, and fleshes out the

  types of text fields that are styled
Lightens the backgrounds, so the contrast is better

It also adds a mixin function for setting the widths of the page, so

a consumer can increase/decrease the width of the navigation pane and

forms areas.

Tested all the config forms pages in Review Board and RBCommons, looking

for regressions. Compared to the previous styling.