fix meta tag for twitter cards and image attachments

The name of the tag should be twitter:image, not twitter:player.

Also, add twitter:image:alt meta tags.

lib/pleroma/web/metadata/providers/twitter_card.ex

@@ -76,9 +76,11 @@ defp build_attachments(id, %{data: %{"attachment" => attachments}}) do
                 {:meta, [name: "twitter:card", content: "summary_large_image"], []},
                 {:meta,
                  [
-                   name: "twitter:player",
+                   name: "twitter:image",
                    content: MediaProxy.url(url["href"])
-                 ], []}
+                 ], []},
+                {:meta, [property: "twitter:image:alt", content: truncate(attachment["name"])],
+                 []}
                 | acc
               ]
               |> maybe_add_dimensions(url)
@@ -130,4 +132,10 @@ defp maybe_add_dimensions(metadata, url) do
         metadata
     end
   end
+
+  defp truncate(text) do
+    # truncate to 420 characters
+    # see https://developer.twitter.com/en/docs/twitter-for-websites/cards/overview/markup
+    Pleroma.Formatter.truncate(text, 420)
+  end
 end