a8n: Add setting to allow read access to Campaigns for non-site-admins
Created by: mrnugget
This should fix https://github.com/sourcegraph/sourcegraph/issues/8013
There is still a lot of things missing here:
- We need to add the new check to every read resolver (that means: CampaignResolver, CampaignPlanResolver, etc.)
- We need to hide buttons/actions in the frontend if the user is allowed to see things
- We need to test that it works (I blind-coded this)
Please add all the necessary changes in this PR so we can easily merge and ship it in a patch release