Org editors and writers should be allowed to manage world documents

This *might* change back again once world is live and we can settle down the permissions properly.  For now, we don't want to disrupt the production of content ahead of the launch so we just allow it.  This does mean that the WorldEditionRules is almost redundant, but that might change.  Of course, if it doesn't then we should remove it and simplify.
Remove world priorities and news from search

They won't get added back in due to 2ae3439, but we also need existing
ones to be removed.
Merge pull request #367 from alphagov/org-writer-and-editor-can-manage-world-documents

Org editors and writers should be allowed to manage world documents

https://www.pivotaltracker.com/story/show/45770559