Admins may want to have discussions or any other general activity
that does not have any particular speaker. Thus, speaker validation
is removed.
Closes https://github.com/openSUSE/osem/issues/2520
Also, in events#show and proposals#index
It is more relevant to check if the speakers have registered to the
conference and filled their biographies than the submitter, because,
there can be multiple speakers and the submitter isn't necessarily one
of them
And, remove admin/events/reports.html.haml, since, it isn't used anymore
Fix#1477, fix#1479
