Cleanup campaign connection resolver

Created by: eseliger

This PR does two things (reviewable in separate commits):

• Move campaigns.go to two separate files, the connection and the node, like we do with most other entities now, and add an additional test that pagination works properly
• Apply the cursor pagination pattern from changeset specs to campaigns, which makes them more aligned and gives us a performance benefit when looking through a bigger list of campaigns

Also: Enjoy our variety of different arguments accepted to the same connection resolver :)