Python path setting (#2409)

* Add python package
* Add python path backend config
* Add python path to system settings page
* Apply python path to script scrapers and plugins

pkg/scraper/xpath_test.go

@@ -830,6 +830,10 @@ func (mockGlobalConfig) GetScraperExcludeTagPatterns() []string {
 	return nil
 }
 
+func (mockGlobalConfig) GetPythonPath() string {
+	return ""
+}
+
 func TestSubScrape(t *testing.T) {
 	retHTML := `
 	<div>