Paginate all, individual and company contributors

The contributors lists are getting a bit long and costly when rendering
the page so let's paginate them.
Refactor User.authorized_contributors to return an AR Relation

So the result of User.authorized_contributors can be paginated using
Kaminari's SQL pagination refactor User.authorized_contributors to
return an ActiveRecord::Relation instead of an Array.
Merge pull request #624 from opscode/paginate-contributors

Paginate all, individual and company contributors