Make sure all public facing controllers are PublicFacingControllers

- html_attachments
- placeholder

Excluding the file attachments controllers for now.
Add test coverage for caching of attachments

Mainly so we have regressions to ensure we don't ever cache
editor-previewed attachment requests
Merge pull request #1142 from alphagov/public-facing-controllers

Make sure all public facing controllers are PublicFacingControllers

https://www.pivotaltracker.com/story/show/61651246