Un-revert Toby's position import change, make stage importing idempotent with respect to script_level positions

The PR #664 was initially reverted because on production it resulted in script levels with null posoitions.

The theory is that the situation on production matches the test case "script import is idempotent w.r.t. positions and count" where the script import has happened multiple times

The fix is to `script_level.insert_at(0)` before `script_level.move_to_bottom`, adding a position to script levels without them.