New themable base theme

I have only touched the base theme. Let us decide on what to do with it before tackling bootstrap3 and friends. Galleries are untouched.

Themes can be created by only only shipping stylesheets. Everything is semantically structured and there are class names (by page kind or general) on every major section.  For example, comments sections can be styled in a more generalized way regardless of the comment system being used.

The site banner is separated out into base_header.tmpl and the footer to base_footer.tmpl. Allowing themes to be strip out headers, navigational menus, and footer without having to make changes to main content data or structure of base. (A sidebar can be injected at the top of base_footer.tmpl, for example.)

fixes #1123