New flag for unnumbered HTML versions
Simplify heading numbering code

The previous code was bending my head. We only ever number h2 and h3
tags, so just number those and keep things simple
govspeak helper supports manual heading numbering
Wire up unnumbered HTML versions on public site
Simplify options for govspeak_to_html
Tidy up admin UI for unnumbered HTML versions

A sprinkling of markdown help for when a user has specified that this
is a manually numbered HTML version
Fix manually numbered HTML version govspeak headers

We don't want to include the decimal list style on manually numbered
html versions' govspeak headers as they won't necessarily tally with
the manual numbering. Instead, we strip out the numbers from the
headings when they've been manually numbered to stop the govspeak
header links looking weird.
Simplify number extraction method
Merge pull request #786 from alphagov/53738105-manual-html-pub-numbering

53738105 manual html pub numbering

https://www.pivotaltracker.com/story/show/53738105