Change ctrl s to download .scn file instead of json

[lalalune]

Describe your changes

Saves a .scn when user presses ctrl s, which can be easily re-uploaded. Requested by creators to speed up development workflow and use the app as an editor.

What are the steps for a QA tester to test this pull request?

Press ctrl s
Scene should be saved
Verify that scene works by dragging back into window to load

Issue ticket number and link

#3652

Checklist before requesting a review

• I have performed a self-review of my code
• I am not adding any irrelevant code or assets
• I am only including the changes needed to implement the change
• I have playtested and intentionally tried to find error cases but couldn't

[avaer]

This is overall good, but given that appManager is already performing a formatting job, IMO it makes sense to move this formatting to appManager.exportJSON.

I don't think there are any other users of that method (it is not how we sync anything).

[lalalune]

Closed in favor of Upstreet PR