base: footer modularity improvement

* Removing styles on the `<footer>`. It's a generic element that may be used at more than one place, like `<p>` or `<address>`.
  See: http://www.whatwg.org/specs/web-apps/current-work/#the-footer-element
* Adding blocks in the footer template so one can reuse the content without having to keep the outter structure.
* Removing inline style.
* `url_for('')` -> `url_for('search.index')`

Usage sample:
* https://github.com/greut/cds-demosite/blob/309f9db351dbc0003955d620b57893670b851fbf/cds/base/templates/footer.html
* redefining it from scratch should still work as expected: https://github.com/inspirehep/inspire-next/blob/new-templates/inspire/base/templates/footer.html

Ping @giokokos for review and comments.