Style HTML attachment contacts

- Add some styling to html publication contacts.
- Let the govspeak helper methods accept a heading level for the
  contacts. This was needed due to the auto number level inserter
- Remove texture from heading number backgrounds as it isn't used on the
  main page anymore.
Merge pull request #623 from alphagov/htmlpub-contacts

Style HTML attachment contacts

https://www.pivotaltracker.com/story/show/52444523