Batch changes site admin configuration page

Created by: malomarrec

User problem

Site-admins get confused or forget to setup batch changes, resulting in frustration and bad first-impressions

See:

• Users confused with credentials setup https://github.com/sourcegraph/sourcegraph/issues/25394
• Users forget to configure webhooks https://github.com/sourcegraph/sourcegraph/issues/21890 https://github.com/sourcegraph/sourcegraph/issues/24312
• Users don't know they can setup rollout windows

Proposal

1. Users can view the important settings in a page (including the status of settings that site admins need to change)
2. Site admins view all the important Batch Changes settings in a page

Site-admin

• Credentials: Batch Changes can either use your account, or a service account. Configure at least one:
• Webhooks
• Forks (optional)
• Rollout Windows (optional)