Prunes unused assets when exporting to a file.
### Change Type
- [x] `patch` — Bug Fix
### Test Plan
1. Insert an image.
2. Delete the image.
3. Save to file.
4. The saved file should not have the asset record present.
5. The file size should also be quite small (around 2kb for a file with
no shapes).
### Release Notes
- Optimize file size of exported files.
0c9e8d323e