Add title component

Add a component for page titles. The title should be large by default
with an option to make it smaller for longer titles.

On average the titles on government bits of content are twice as long as
those found on general guidance. We shrink the font size down so that
90% of titles will fit onto two lines.

Note: This builds on the work in #486 as that converts the docs to YAML
rather than JSON. That pull request should be merged before this one.