WIP base: removal of demosite-specific stylesheets

Base css file should contain just framework specific rules and be easily extendible e.g by less files with customisations.

Much of base.less file contains style rules related to navigation bar, footer and general layout of demosite. This makes impossible inheriting and including this file in base stylesheet bundle for an overlay.

Please look again if I didn't throw away some important stuff. There are two places where the deleted rules can go:
1. A separated file in invenio - the style rules are apparently related to some of html templates which exist in invenio itself, especially the home page.
2. A css bundle for demosite.

Please decide what to do with the deleted styles and where should I move them from base.less.
General fix for fixed navbar. You can override it in your overlay if you want change default menu style.