Allow system migrations to access unpruned actor and item sources

[In3luki]

This is the only way I was able to make it work reliably.

• Move the check for necessary migrations before Documents are initialized.
• Record the unpruned source in a seperate property on actors and items only if a migration is necessary.
• Use the unpruned source for migrations if it's available.

I've successfully migrated a 2 year old backup of my test world multiple times. Including some test runs where I migrated a property that was no longer in the current data schema.
There still might be a better way to do this but I am out of ideas.