Fix preview

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/service/metadata/event.ex

@@ -6,7 +6,11 @@ defimpl Mobilizon.Service.Metadata, for: Mobilizon.Events.Event do
   alias MobilizonWeb.MediaProxy
 
   def build_tags(%Event{} = event) do
+    event = Map.put(event, :description, process_description(event.description))
+
     tags = [
+      Tag.content_tag(:title, event.title <> " - Mobilizon"),
+      Tag.tag(:meta, name: "description", content: event.description),
       Tag.tag(:meta, property: "og:title", content: event.title),
       Tag.tag(:meta, property: "og:url", content: event.url),
       Tag.tag(:meta, property: "og:description", content: event.description),
@@ -33,6 +37,13 @@ defimpl Mobilizon.Service.Metadata, for: Mobilizon.Events.Event do
       ]
   end
 
+  defp process_description(description) do
+    description
+    |> HtmlSanitizeEx.strip_tags()
+    |> String.slice(0..200)
+    |> (&"#{&1}…").()
+  end
+
   # Insert JSON-LD schema by hand because Tag.content_tag wants to escape it
   defp json(%Event{} = event) do
     "event.json"