Refactor AnonymousContact#find_all_starting_with_path

The new version is a pure ActiveRelation scope and can be
chained (eg for pagination)
Set path for all anonymous contacts
Add pagination of anon feedback using kaminari
Remove client-side table sorting

- This feature doesn't make sense in a paginated world
- It also wasn't very useful
Merge pull request #159 from alphagov/paginate-anonymous-feedback

Paginate anonymous feedback