Stop special-casing sub-org about page URLs.

Sub-organisations no longer show their About Us content on their home page, so
this changes the public document helper to correctly return the /about URL for
these pages. Also fixes the issue that about pages were overwriting the org home
page in the search index.
Merge pull request #1549 from alphagov/fix_suborg_urls

Stop special-casing sub-org about page URLs.