Fix displaying follow notifications from Mastodon

Mastodon, unlike Pleroma, doesn't include `published` in the Follow
activity, so we fall back to the DB's inserted_at.

lib/clacks_web/templates/frontend/_follow_notification.html.eex

@@ -4,7 +4,7 @@
       Followed by <a href="<%= local_actor_link(@actor) %>"><%= @actor.data["preferredUsername"] %></a>
     </p>
     <p class="notification-info-right">
-      <time datetime="<%= @activity.data["published"] %>"><%= display_timestamp(@activity.data["published"]) %></time>
+      <time datetime="<%= @activity.data["published"] %>"><%= display_timestamp(@activity) %></time>
     </p>
   </div>
 </div>

lib/clacks_web/views/frontend_view.ex

@@ -35,13 +35,20 @@ defmodule ClacksWeb.FrontendView do
   def local_actor_link(%Actor{local: false, id: id}),
     do: Routes.frontend_path(Endpoint, :actor, id)
 
-  @spec display_timestamp(datetime :: String.t() | DateTime.t() | NaiveDateTime.t()) :: String.t()
+  @spec display_timestamp(
+          datetime :: String.t() | DateTime.t() | NaiveDateTime.t() | Activity.t()
+        ) :: String.t()
 
   def display_timestamp(str) when is_binary(str) do
     display_timestamp(Timex.parse!(str, "{ISO:Extended}"))
   end
 
-  def display_timestamp(datetime) do
+  def display_timestamp(%Activity{data: data, inserted_at: inserted_at}) do
+    display_timestamp(Map.get(data, "published", inserted_at))
+  end
+
+  def display_timestamp(%{__struct__: struct} = datetime)
+      when struct == DateTime or struct == NaiveDateTime do
     diff = Timex.diff(Timex.now(), datetime, :seconds)
 
     cond do