Show event time in conference timezone

Added an application helper method `time_with_timezone` to return time with the conference timezone which solves the issue of wrong timezone in `scheule.xml` and `admin/events#show`.

Fixes #1188

app/helpers/application_helper.rb

@@ -34,6 +34,11 @@ module ApplicationHelper
     result
   end
 
+  # Returns time with conference timezone
+  def time_with_timezone(time)
+    time.strftime('%F %R') + ' ' + @conference.timezone.to_s
+  end
+
   ##
   # Checks if the voting has already started, or if it has already ended
   #