Support full backups with attachments for instances with large storage usage

Issue Summary

When you have an instance with a large storage usage of attachments, say 1TB+, it is currently impossible to create and export the full backup (including attachments) from Cloud. It is only possible to export it without attachments.  

Steps to Reproduce

1. In an instance with large storage usage, go to the Back up manager , under Back up for cloud, check the box "Include attachments, avatars, and logos in the backup".
2. Select "Create backup for cloud"

Expected Results

The backup may take hours or days (depending on the storage usage), and it is comprehensible due to its sheer size, but it will eventually finish, and you'll be able to download it. 

Actual Results

After hours or days (depending on the storage usage), the following error will be thrown: 

Workaround

It is not the ideal solution, but a sandbox instance could be used as a backup*, you would need to create a sandbox and then use the 'Copy production data' https://support.atlassian.com/organization-administration/docs/manage-product-sandboxes/#Copy-data-to-a-sandbox 
*Note that there are some components that are not copied to the sandbox.