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.
Yes it is!

Btw: I disabled the js errors for capybara because we have some errors in the proposal edit view (https://github.com/openSUSE/osem/blob/master/app/views/proposal/_form.html.haml#L37). The inline js is really a mess :( But if I remember correct @hennevogel already decided that we will remove the Event Attachment feature so I didn't put effort in it to locate the error! Maybe it's now the right time to remove it ...