Add more detail to OpenGraph tags

Closes #1073

app/helpers/open_graph_helper.rb

@@ -0,0 +1,16 @@
+module OpenGraphHelper
+  def opengraph_tags(title = nil)
+    tags = {
+      "og:site_name" => t("layouts.project_name.title"),
+      "og:title" => [t("layouts.project_name.title"), title].compact.join(" | "),
+      "og:type" => "website",
+      "og:image" => image_path("osm_logo.svg", :host => SERVER_URL),
+      "og:image:width" => "200",
+      "og:image:height" => "200"
+    }
+
+    tags.map do |property, content|
+      tag(:meta, :property => property, :content => content)
+    end.join("").html_safe
+  end
+end

app/views/layouts/_head.html.erb

@@ -26,7 +26,7 @@
   <%= tag("link", { :rel => "publisher", :href => "https://plus.google.com/111953119785824514010" }) %>
   <%= tag("link", { :rel => "search", :type => "application/opensearchdescription+xml", :title => "OpenStreetMap Search", :href => asset_path("osm.xml") }) %>
   <%= tag("meta", { :name => "description", :content => "OpenStreetMap is the free wiki world map." }) %>
-  <%= tag("meta", :property => "og:image", :content => image_path("osm_logo.png")) %>
+  <%= opengraph_tags %>
   <% if flash[:piwik_goal] -%>
   <%= tag("meta", :name => "piwik-goal", :content => flash[:piwik_goal]) %>
   <% end -%>