Add basic cache directives to gitlab-pages

Per https://gitlab.com/gitlab-com/infrastructure/issues/2082#note_33664491

Cache-Control: max-age=600 and Expires: time.now()+10m. This is what is being used by GitHub Pages.

We can add this globally for now, notwithstanding the desire in #50 to be able to control this per-project.

mentioned in issue gitlab-com/infrastructure#2082 (closed)

mentioned in merge request !35 (merged)

closed via merge request !35 (merged)

mentioned in issue #77 (closed)