Fix up header and generally make css more consistent

This PR makes some changes mostly to the css but a little but to the templates, aimed at getting a site out that's consistent in the header (and somewhat moreso style) with the astropy web pages).

You can see the result at http://eteq.github.io/astropy-tutorials/

Note that this is also intended to replicate the look of http://eteq.github.io/test-gh-pages/ , which will probably soon be data.astropy.org

Look good, @adrn @astrofrog