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.
Because we can't rely that we will have conference / commercial model in future? @hennevogel told me so ...

In this Blog article it's also described 
http://railsguides.net/change-data-in-migrations-like-a-boss/#writing-code-in-migrations-without-caution

But maybe I understand something wrong, please explain your view :)