Add thumbnail image URL property to Product class

[jsauvexamarin]

Memory usage seems high on list screens with thumbnail images, because the full size image files are being used. This can be mitigated by using actual thumbnail image files.

1. Add the thumbnail image url property to the product and category classes.
2. Create thumbnail image files for each full size image.
3. Upload thumbnail images to Azure blob storage.
4. Reference thumbnail image files in the service's data initializer.
5. Re-run the data initializer.