Add support for twitter cards and facebook shares

Added missing image url as well as created twitter card.
validated both with twitter validator and facebook validator.
All passed.

Need to check to see if no image is selected, what is outcome.

app/controllers/conferences_controller.rb

@@ -28,6 +28,8 @@ class ConferencesController < ApplicationController
       redirect_to admin_conference_splashpage_path(@conference.short_title) && return
     end
 
+    @image_url = "#{request.protocol}#{request.host}#{@conference.picture}"
+
     if splashpage.include_cfp
       cfps = @conference.program.cfps
       @call_for_events = cfps.find { |call| call.cfp_type == 'events' }

app/views/conferences/show.html.haml

@@ -3,13 +3,19 @@
   %meta{ property: "og:site_name", content: (ENV['OSEM_NAME'] || 'OSEM') }
   %meta{ property: "og:description", content: @conference.description }
   %meta{ property: "og:url", content: conference_url(@conference.short_title) }
+  %meta{ property: "twitter:card", content: "summary_large_image" }
+  %meta{ property: "twitter:image", content: @image_url }
+  %meta{ property: "twitter:title", content: (@conference.title) }
+  %meta{ property: "twitter:description", content: @conference.description }
   - if @conference.picture?
-    %meta{ property: "og:image", content: @conference.picture_url }
+    %meta{ property: "og:image", content: @image_url }
-    %meta{ property: "og:image:secure_url", content: @conference.picture_url }
+    %meta{ property: "og:image:secure_url", content: @image_url }
+
 
 = content_for :title do
   = @conference.title
 
+
 #splash
   - if @conference.code_of_conduct.present?
     = render 'code_of_conduct', organization: @conference.organization
@@ -50,6 +56,7 @@
       sponsorship_levels: @sponsorship_levels,
       sponsors: @sponsors
 
+
   -# footer
   - if @conference.splashpage.include_social_media
     - if @conference.contact.has_social_media?