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/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:secure_url", content: @conference.picture_url }
+    %meta{ property: "og:image", content: @image_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?