More CSS overrides of UberMenu

These are intended to replace those in he UberMenu basic.css that were
added. We don't want to alter the files of plugins, so that we can
upgrade the plugins when necessary.
UberMenu extension

This fixes some really odd functionality, in which the dropdown subnavs
would overlay main menu <li>s on mobile widths.
Adding a sidebar template

This overrides the sidebar in twentytwelve. For pages, it displays a
simple page heirarchy for each "section," allowing for more sideways
navigation by users. It still needs to be stylized some.