Sync service secret storage fixes
The sync service has been broken since we removed D-Bus access to the secret service, because we forgot to port it to the new SecretPassword API, which allows use of the secrets portal.
Hopefully fixes #1233 (closed), although I haven't tested this because the easiest way to test flatpak-related commits is to yolo commit to master and then check if Ephy Tech Preview still works after it is built.
Also: EphySyncService has no protection against async callbacks being executed after it has been destroyed. That's no good. Cancel async calls where required.