added events, confirmed events and events with comments export features with xlsx, pdf and csv format

app/views/admin/events/_confirmed_events.pdf.prawn

@@ -0,0 +1,35 @@
+prawn_document(force_download: true, filename: "#{@file_name}.pdf", page_layout: :landscape) do |pdf|
+  events_array = []
+  header_array = ['Event ID',
+                  'Title',
+                  'Abstract',
+                  'Start time',
+                  'Submitter',
+                  'Speaker',
+                  'Event Type',
+                  'Track',
+                  'Difficulty Level',
+                  'Room',
+                  'State'
+                 ]
+  events_array << header_array
+  @events.confirmed.each do |event|
+    row = []
+    row << event.id
+    row << event.title
+    row << event.abstract
+    row << (event.time.present? ? "#{event.time.strftime("%Y-%m-%d")} #{event.time.strftime("%I:%M%p")} " : '')
+    row << event.submitter.name
+    row << event.speaker_names
+    row << event.event_type.title
+    row << (event.track.present? ? event.track.name : '')
+    row << (event.difficulty_level.present? ? event.difficulty_level.title : '')
+    row << (event.room.present? ? event.room.name : '')
+    row << event.state
+    events_array << row
+  end
+
+  pdf.text "#{@conference.short_title} Confirmed Events", font_size: 25, align: :center
+  pdf.table events_array, header: true, cell_style: {size: 8, border_width: 1}
+
+end