use selectors to test h1 content only on static pages
use shared examples on static_pages spec

There was too much duplication in the static pages spec, since many
features of the static pages are shared.
add nav link tests to spec
add spec for full_title helper
remove duplicate full_title helper from spec utils

Now that we have tests for the application helper full_title,
we don't have any need for the separate and identical spec helper
of the same name. Our specs can reuse the application helper, confident
that it is correct thanks to the testing in the previous commit