Extract #entry_for_content_type helper
Render subtype-less content types as a single option

This means we have a mixture of <option> and <optgroup> tags inside the
content type drop-down, which is probably what we want for now.
Merge pull request #37 from alphagov/content-type-grouping

Content type grouping
Merge commit '492bde638d8b5e76b7decc583d0026e04e3998d0' into HEAD