Refactoring Conference/Event media to new media object

close #420

As in #420 described Conference and Event has now a has_many relation to media.
Hm, no it's intentionally because currently we don't use strong parameters. But I believe we will / have to change to strong parameters some time. As agreed with @hennevogel we don't want to mix different attribute access methods so I simply commented it! But I can remove the comment if you think it would be better ...