Fix attachment ordering with negative values
Wrap reordering in a database transaction

this ensures that if two people try to reorder the same attachments at
the same time, only one set of orderings will be persisted and a
consistent state will remain, rather than possibly two sets of orderings
being confounded and an inconsistent state remaining.
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]
Merge pull request #1085 from alphagov/fix-attachment-ordering-500-error

Fix attachment ordering with negative values

[Fixes https://www.pivotaltracker.com/story/show/61203896]