Enable footnotes for specialist-publisher.

Footnotes were causing errors on publish because the sanitize gem (a
dependency of govspeak) was erroring with invalid HTML when attempting
to use colons in ids.

Upgrade sanitize, then upgrade govspeak, then upgrade
govuk_content_models (depends on govspeak), and now finally upgrade
specialist publishers versions of govspeak and govuk_content_models.

Add help text for footnotes taken from Whitehall.
Merge pull request #209 from alphagov/add-footnotes

Enable footnotes for specialist-publisher.