Move ConfiguredMixin and adapters to a sub-module

Keeping this in buildbot.util was causing problems with re-importing
util in multiprocessing, and anyway was cluttering __init__.py.
s/user_infos/user_info/g

Plus fix docs so they build

+autopep8
use class names as auth names, rather than inventing variations
move SessionConfigResource to its own module, since it's not auth-related
reorg docs to break out auth classes, auth docs, and avatars.
edit auth.rst

This includes a more explicit description of the HTTP transactions, but
is otherwise pretty much the same content.
document server-side session contents
move the now-pretty-big www config docs to their own page
document asyncRenderHelper
edit auth manual bits
add license headers