Geo: Add setting for ActionCable allowed origins

What does this MR do and why?

Add setting for ActionCable allowed origins

This enables admins to setup allowed origins for websocket requests through ActionCable in order to ensure communication between secondary and primary Geo sites via websockets.

To ensure users do not provide a regexp and open the allowed origins widely, the new setting value is filtered to only allow URLs.

References

• Issue: #579074
• Implementation plan: #579074
• Charts MR: gitlab-org/charts/gitlab!4644 (merged)
• Omnibus MR: omnibus-gitlab!8910 (merged)

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