Implement RFC312 Site credentials for Batch Changes

Created by: eseliger

Tasks involved

• Get RFC approved
• Implement site credentials table and store methods
• Make enterprise site admin menu entries configurable
• Implement site credentials UI in admin area
• Use site credentials in reconciler and syncer
• Document deprecation process
• Add documentation on service accounts and where to configure them

RFC issue: https://github.com/sourcegraph/sourcegraph/issues/17919