share every calendar with one user

You can use PowerShell:

$calendars = Get-Mailbox -RecipientTypeDetails UserMailbox | Get-MailboxFolderStatistics -FolderScope Calendar | ? {$_.FolderType -eq "Calendar"} | select @{n="Identity"; e={$_.Identity.ToString().Replace("\",":\")}}

$calendars | % {Set-MailboxFolderPermission -Identity $_.Identity -User Email address removed -AccessRights LimitedDetails}

It's not possible to do this via the admin console, even manually. It's a client-level operation so you will need to grant yourself Full access permissions and open each mailbox in OWA/Outlook first, then open each Calendar and adjust permissions.