Implement `repos.list` replacement

Created by: tsenart

Checklist:

• Add "OtherExternalServiceConnection" type to site.schema.json
• Document changes.
• Update Changelog.
• Add code to https://github.com/sourcegraph/sourcegraph/tree/master/cmd/repo-updater/repos to fetch repos for these other connections.
• Fix JSON schema Monaco editor validation (it doesn't seem to support dependencies)
• Write a test plan

Closes #1550 (closed)