Download .sb3 files with a project thumbnail

[paulkaplan]

The .sb3 file should contain a thumbnail, to make importing into native apps and scratch-desktop more intuitive. A snapshot of the stage at the moment of download would be fine.

We need to check with @kchadha to see if this has any issues with respect to parsing/validating sb3's. We would also need to decide if we want to use that thumbnail in any way when uploading files to the web.

[kchadha]

We need to check with @kchadha to see if this has any issues with respect to parsing/validating sb3's.

Adding files to the .sb3 should not have any issues w/parsing/validation.