Add maintenance mode/admin mode to allow admins to verify things work well post-upgrade
Please see &2149 for the ongoing work to create a maintenance/read-only mode. This issue remains open to preserve the context.
Zendesk issue: https://gitlab.zendesk.com/agent/tickets/29713
Customer would like to have a maintenance mode or admin mode that only allows admins to login and browse GitLab. This would allow them to verify or validate that everything works well post-upgrade before opening it up to the masses. We currently have the deploy page but this does not allow admins to access GitLab for this purpose.