lib/model, lib/versioner: Drive version cleanup from scanner (fixes #6313) (#6693)

This change adds a separate config for the cleanup interval, and runs that cleanup from the main folder loop.

lib/versioner/external.go

@@ -7,6 +7,7 @@
 package versioner
 
 import (
+	"context"
 	"errors"
 	"os"
 	"os/exec"
@@ -115,3 +116,7 @@ func (v external) GetVersions() (map[string][]FileVersion, error) {
 func (v external) Restore(filePath string, versionTime time.Time) error {
 	return ErrRestorationNotSupported
 }
+
+func (v external) Clean(_ context.Context) error {
+	return nil
+}