Improvements to creating batch changes on forks

Created by: malomarrec

This tracks potential follow-up work for the fork MVP #17879 (closed).

Future work:

• Fork-settings are linked to credentials. Each PAT or service account token has a setting to use forks or not. Site-admins can override this, and enforce that all changesets created by Sourcegraph are on forks.
• Should there be a "default to fork if permission error". As a user, provided there is no site-admin level fork-setting, I can set up Batch Changes to create a fork instead of a branch if creating a changeset errors due to permission issues. Blocked by: Sync write-access permission #24307