Stream ZIP exports to S3

[jnm]

Right now, the ZIP export process looks like:

1. Download every single media attachment from S3;
2. Add it to a ZIP file in /tmp/;
3. Upload the ZIP file to S3;
4. Return the result to the user.

These temporary ZIP files are often exhausting our servers' disk space and causing many wasted hours. It's time to stop the madness.

[jnm]

In our sights:

kobocat/onadata/libs/utils/export_tools.py

Lines 843 to 847 in 7957abd

	with NamedTemporaryFile('wb+', prefix='media_zip_export_', suffix='.zip') as temporary_file:
	create_attachments_zipfile(attachments, temporary_file=temporary_file)
	export_filename = get_storage_class()().save(
	file_path,
	File(temporary_file, file_path))

[jnm]

I think I'm hitting this again: jschneier/django-storages#449

[jnm]

Will need to use my fork until jschneier/django-storages#504 is merged.