add a hackish migration for the changes expiration index.

database/mongodb/changes.go

@@ -80,6 +80,9 @@ func (r *changeRepository) Remove(ctx context.Context, change models.Change) err
 func newChangeRepository(db *mgo.Database) (repositories.ChangeRepository, error) {
 	collection := db.C("common.changes")
 
+	// Delete the old index if it exists.
+	_ = collection.DropIndexName("date_1")
+
 	err := collection.EnsureIndex(mgo.Index{
 		Name:        "expiry",
 		Key:         []string{"date"},