MastoAPI: Only return create activties in contexts.

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

@@ -196,6 +196,7 @@ def get_context(%{assigns: %{user: user}} = conn, %{"id" => id}) do
     with %Activity{} = activity <- Repo.get(Activity, id),
          activities <- ActivityPub.fetch_activities_for_context(activity.data["object"]["context"], %{"blocking_user" => user}),
          activities <- activities |> Enum.filter(fn (%{id: aid}) -> to_string(aid) != to_string(id) end),
+         activities <- activities |> Enum.filter(fn (%{data: %{"type" => type}}) -> type == "Create" end),
          grouped_activities <- Enum.group_by(activities, fn (%{id: id}) -> id < activity.id end) do
       result = %{
         ancestors: StatusView.render("index.json", for: user, activities: grouped_activities[true] || [], as: :activity) |> Enum.reverse,