Add support for remote (cloud) file systems

Currently, the gateway only supports reading & writing files to the local file system. However, some users have requested the possibility to save and retrieve files from cloud platforms (like S3, Azure or Google Cloud).

Thus, the gateway should be modified to allow users to save and retrieve files from/to a cloud platform.

In this first step, support should be added for the following platforms (other platforms can always be added later):

Amazon S3
Microsoft Azure
Google Cloud
Openstack

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information