This commit:
1. Updates all main templates to use the new `.header-title` class,
making these titles less disruptively large on mobile devices.
2. Centers these titles for mobile breakpoints only, re-setting them
back to left-aligned text for `md-` breakpoints and up.
Note that in `lib/gollum/templates/wiki_contentf.mustache`, the
`.header-title` is not a `#head .header-title`, so it doesn't inherit
all of the new styles added in this commit.
benjamin wil
4fcc72078b