replace backend var semester_aktuell with api parameter in grades, student and status endpoints, adapt old and new js api definitions, adapt frontend, add validation method to studiensemester_model

application/models/organisation/Studiensemester_model.php

@@ -341,4 +341,10 @@ class Studiensemester_model extends DB_Model
 		WHERE studiensemester_kurzbz = ?",[$studiensemester_kurzbz]);
 
 	}
+
+	public function isValidStudiensemester($studiensemester_kurzbz)
+	{
+		$result = $this->load($studiensemester_kurzbz);
+		return hasData($result);
+	}
 }