Revert "Revert "Merge pull request #664 from code-dot-org/script-stages-empty""

This reverts commit 5b36a1865485b331079cafbb1c749bb3d52342b1.
Handle multiple script import positionings correctly
Attempt to repro stage gap creation
Add note on test
Support empty stages and stage reordering
Add simple jigsaw visit test
Merge pull request #694 from code-dot-org/consistent_stage_numbering

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