Show inline attachments on worldwide org pages.

Use the govspeak_edition_to_html helper, which requires the whole Edition object
rather than just the body. To enable this, make HasCorporateInfo.about_us a
public method.
Prevent errors being caused by nil parameter

Not all worldwide organisations will have an about page, or at least
they won't until one is created.
Merge pull request #1640 from alphagov/worldwide_cips

Show inline attachments on worldwide org pages.