Bump requirement on django-classy-tags

For some reason the requirement in django-cms for
django-classy-tags>=0.5 isn't being respected -- probably because we're
installing from git.

This newer version of django-classy-tags is required for the correct
behaviour of

    {% page_url "foo" as foo_url %}
Correct header height calculations

These were messed up for two distinct reasons:

- The new icon is 50px high, and the font-size + padding of the h1 came
  to less than 50px.
- We were calculating the padding on the social icons on the basis that
  they were 24px high. The new glyphicon things are actually 28px high.
Improve nav behaviour & display on small screens

- The search box now sizes itself down on the small ("sm") screen size.
- The secondary nav is collapsed into the main nav on "xs" screen sizes.
- The nav menu doesn't scroll when it grows past 340px high.