Merge branch 'feature/backup'

Merge the new backup system into master. The new backup is not complete
and still requires polishing and some other work, but it is in a lot
better condition that the one in master. Hence the master.

Additionally, this moves around certain code shared with the storage
service a lot, and that makes merges harder.

Conflicts:
services/storage/syncresourceidentifier.h
services/storage/syncresourceidentifier.cpp