Fix article/pages not overriding html title

Due to a recent Jinja update, templates that extend `base.html` can't
override `title` block the included `head.html` anymore. This commit
simply moves everything from head.html to base.html, allowing overriding
again.
Merge pull request #20 from nhanb/cope-with-jinja-update

Fix article/pages not overriding html title