Add follow notifications.

lib/pleroma/notification.ex

@@ -20,7 +20,7 @@ def for_user(user, opts \\ %{}) do
     Repo.all(query)
   end
 
-  def create_notifications(%Activity{id: id, data: %{"to" => to, "type" => type}} = activity) when type in ["Create", "Like", "Announce"] do
+  def create_notifications(%Activity{id: id, data: %{"to" => to, "type" => type}} = activity) when type in ["Create", "Like", "Announce", "Follow"] do
     users = User.get_notified_from_activity(activity)
 
     notifications = Enum.map(users, fn (user) -> create_notification(activity, user) end)

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

@@ -145,6 +145,8 @@ def notifications(%{assigns: %{user: user}} = conn, params) do
         "Announce" ->
           announced_activity = Activity.get_create_activity_by_object_ap_id(activity.data["object"])
           %{id: id, type: "reblog", created_at: created_at, account: AccountView.render("account.json", %{user: actor}), status: StatusView.render("status.json", %{activity: announced_activity})}
+        "Follow" ->
+          %{id: id, type: "follow", created_at: created_at, account: AccountView.render("account.json", %{user: actor})}
         _ -> nil
       end
     end)