Allow to delete accounts from synchronization backend

[mtotschnig]

When an account that has been synced is deleted locally, it continues to exist on the back-end. Since the folder is named by uuid, it might not be easy to identify which folder to delete using the WebUI of the cloud provider. For non-encrypted backends, you can open the metadata json file to find out which account the folder represents, but this is not possible on encrypted backends.
We should provide an option in the app to delete an unsynced account on the backend.