Enable an extra level of heading numbers

There is a requirement to add another layer of heading numbers to better
structure the documents. This can now support one or two levels. It
could be possible to exetend this to handle any number though.
Henry's styling requests

- whitespace around h2 .number mobile view
- whitespace around h3 .number
- figcaption styling
- fix th bold style
- whitespace style for p, ul and ol nested in table cells
- move whitespace character inside .number
Fix broken test
Merge pull request #448 from alphagov/heading-numbers

HTML Publication Heading Numbers