Use filter to set analytics headers on documents

Setting them explicitly on each controller meant they were getting
missed from various formats. We always want to set org analytics
headers, so do it in DocumentsController.

https://www.pivotaltracker.com/story/show/61487628
Set org analytics headers on activity pages

https://www.pivotaltracker.com/story/show/61487628
Order edition org analytics header by importance

https://www.pivotaltracker.com/story/show/61487628
Add analytics headers to org about pages

https://www.pivotaltracker.com/story/show/61487628
Order topics org analytics header by importance

https://www.pivotaltracker.com/story/show/61487628
Merge pull request #1157 from alphagov/61487628-fix-analytics-organisation-headers

Fix analytics organisation headers

https://www.pivotaltracker.com/story/show/61487628