Added unit test for SafeHtmlValidator
Footnote only allowed on Publication HtmlAttachments

Commit also modifies SafeHtmlValidator to set @record instead
of passing it round as method arguments.
Footnotes are styled
Govspeak help includes footnotes where appropriate
Publication doesn't allow footnotes in body
Require govspeak version ~> 0.2.4

Because that version allows sanitize version 2.0.6
Correct markup indentation in _govspeak_help partial
Split out footnote tag validation into NoFootnotesInGovspeakValidator

Footnotes are explicitly banned in most models.
All types of HtmlAttachment can now have footnotes as a wanted side effect.
Merge pull request #1069 from alphagov/enable-footnote-markdown-in-html-pubs

Enable footnote markdown in html pubs

[Delivers https://www.pivotaltracker.com/story/show/60399700]