Make SSBC available to non-site-admins

Created by: eseliger

• Allow non-site-admins in resolvers https://github.com/sourcegraph/sourcegraph/pull/28671
• Don't allow access to cache entries from different users
• Non-site-admins should not be able to see shallow batch changes created by other users - https://github.com/sourcegraph/sourcegraph/pull/28726