Private cloud repositories should use hashed identifiers instead of repository names

Created by: muratsu

As per RFC 520 REVIEW: Browser extension fallback telemetry and design brief, we're in the process of migrating to using hashed identifiers for private cloud repositories.

Please link all the related work items to this parent tracking issue.

• https://github.com/sourcegraph/sourcegraph/issues/27382
• https://github.com/sourcegraph/sourcegraph/issues/28070
• Make a release with interim ☝️ fix
• Confirm hashed repo name check solution w/ security and repo-management teams
• Merge https://github.com/sourcegraph/sourcegraph/pull/28387
• Merge https://github.com/sourcegraph/sourcegraph/pull/28621
• Deploy Browser Extension