Move manually_numbered_headings to GovspeakContent

We need to re-compute the HTML when the numbering scheme changes, otherwise
we'll be serving stale HTML.
Mark old columns as deprecated

This will prevent ActiveRecord from throwing any hissy fits when they are
eventually dropped due to it trying to update/set non-existent columns.
Gracefully handle re-editioning during interim

This is some more temporary code to handle the state between the code being
deployed and the data migration to move the body to the new model has completed.
If a user should re-edition a document during this period where a HtmlAttachment
doesn't have an associated GovspeakContent instance (i.e. before the data
migration has completed), then we want to clone the attachment and manually
create the new assocation. This will be removed in the cleanup pull request to
come later.
Remove old, unused code

This data hygiene code was relying on columns that will no longer exist, which
was causing the build the break.