Replace submitter with speakers in reports#index

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

app/models/event.rb

@@ -214,9 +214,9 @@ class Event < ActiveRecord::Base
   # Returns +Hash+
   def progress_status
     {
-      registered: program.conference.user_registered?(submitter),
+      registered: speakers.all? { |speaker| program.conference.user_registered? speaker },
       commercials: commercials.any?,
-      biography: !submitter.biography.blank?,
+      biographies: speakers.all? { |speaker| !speaker.biography.blank? },
       subtitle: !subtitle.blank?,
       track: (!track.blank? unless program.tracks.empty?),
       difficulty_level: !difficulty_level.blank?,