Make tutorials monotonic.

[PFedak]

Don't reshow tutorial steps that have been completed.

Also make video tutorials respect the tutorial setting.

[PFedak]

Fixes #1126, which seems like the only case people are likely to encounter/be annoyed by. I can see some value in repeating the steps for the very first tutorials if e.g. someone accidentally disconnects their conveyor from the hub. Maybe just the completion state should be sticky.