[FIX] Use proper email subjects and add pay link
[FIX] Use proper email subjects and add pay link

- Do not use "new registrant" subject in payment emails
- Include payment link in payment summary email

DirkHoffmann/indico,pferreir/indico,indico/indico,OmeGak/indico,mvidalgarcia/indico,ThiefMaster/indico,DirkHoffmann/indico,mic4ael/indico,ThiefMaster/indico,OmeGak/indico,ThiefMaster/indico,mvidalgarcia/indico,pferreir/indico,mic4ael/indico,OmeGak/indico,mic4ael/indico,ThiefMaster/indico,mvidalgarcia/indico,mvidalgarcia/indico,pferreir/indico,DirkHoffmann/indico,mic4ael/indico,OmeGak/indico,DirkHoffmann/indico,indico/indico,indico/indico,pferreir/indico,indico/indico