Feature: amendments

@emanuelschuetze the backend is done. I added the relevant links into the template. Could you make it pretty?
This prefix ("A") has to be configurable as config input field. (default in German: "Ä" like "Änderungsantrag").
And please add "-" befor prefix, e.g. main motion "1" and amendment motion "1-Ä" (without blanks) or "1-Ä2" for second amendment motion.