Add AllSync WebDAV support

#1

Piggybacking off of Add Support for AllSync, I investigated and found this:
https://support.allsync.com/en-us/article/39-how-can-i-backup-my-content-from-other-platforms-to-allsync
which seems to suggest they have a WebDAV backend. I don’t even use the service, but thought this might be relevant.