Files are deleted from S3 before a model instance is saved

[oliverroick]

Steps to reproduce the error

1. Create a resource or upload an XLSForm to a project and save.
2. Edit the resource or project and click "remove" next to the file field.
3. Cancel editing.
4. Try to access the file on S3.

Actual behavior

The file is deleted from S3, whether the model instance is saved or not. If the user removes the file from the field and then cancels editing, the database will still have the link but the resource does not exist.

Expected behavior

The file should only be deleted when the model instance is actually saved. That means removing the file from S3 should be moved from django-buckets into the respective model. Before saving we need to check whether the file was removed and then delete the file from S3. If the file was replace with a new file, the old file should be removed.

Currently, this affects the following models (probably not exhaustive):

• Project (for XLSForms)
• Resource