Performer custom fields (#5487)

* Backend changes
* Show custom field values
* Add custom fields table input
* Add custom field filtering
* Add unit tests
* Include custom fields in import/export
* Anonymise performer custom fields
* Move json.Number handler functions to api
* Handle json.Number conversion in api

pkg/sqlite/repository.go

@@ -222,8 +222,8 @@ func (r *repository) innerJoin(j joiner, as string, parentIDCol string) {
 }
 
 type joiner interface {
-	addLeftJoin(table, as, onClause string)
-	addInnerJoin(table, as, onClause string)
+	addLeftJoin(table, as, onClause string, args ...interface{})
+	addInnerJoin(table, as, onClause string, args ...interface{})
 }
 
 type joinRepository struct {