Refactor seeding of indexes with index_prefix
Refactor seeding of indexes with index_prefix

Having this in HTMLBuilder.generate_html places the control in a single
place rather than distributing it through HTMLBuilder subclasses.

Thanks to @cmc333333 for the suggestion.

18F/regulations-site,eregs/regulations-site,tadhg-ohiggins/regulations-site,tadhg-ohiggins/regulations-site,tadhg-ohiggins/regulations-site,18F/regulations-site,eregs/regulations-site,eregs/regulations-site,eregs/regulations-site,18F/regulations-site,tadhg-ohiggins/regulations-site,18F/regulations-site