Add Borg as backup option

[jdaviescoates]

https://borgbackup.readthedocs.io/en/stable/

Some related threads:
https://forum.cloudron.io/post/10547
https://forum.cloudron.io/post/15574

[infogulch]

borg2 is in the works which reorganizes the storage backend to support any kind of remote storage, recently including everything supported by rclone.