sync: Add Init and Sync functions

Init initializes the directory/repository before the first use.
Sync is called to unsure the directory is up-to-date.

admin/sync/importer_cloud.go

@@ -38,6 +38,14 @@ func (i CloudImporter) Kind() string {
 	return "cloud file importer: " + i.baseDAV.String()
 }
 
+func (i CloudImporter) Init() error {
+	return nil
+}
+
+func (i CloudImporter) Sync() error {
+	return nil
+}
+
 func (i CloudImporter) exists(filename string) bool {
 	fullURL := i.baseDAV
 	fullURL.Path = path.Join(fullURL.Path, filename)