`authorized_keys_enabled` is `nil` by default

ZD: https://gitlab.zendesk.com/agent/tickets/79236

It appears that authorized_keys_enabled is off by default, which breaks every EE user who relies on this functionality.

We need to set this to true by default (disable for GitLab.com) and rebuild SSH keys.

This needs to happen ASAP in a patch release.

/cc: @mkozono, @DouweM, @ClemMakesApps, @kushalpandya