Fix article/pages not overriding html title

Due to a recent Jinja update, templates that extend `base.html` can't
override `title` block in the included `head.html` anymore. This commit
simply moves everything from head.html to base.html, allowing overriding
again. See mitsuhiko/jinja2@59713a3acfa0afeac2ee572d213680bbb295280b