Fold RequireLogin plug into Auth module

lib/music_library_web/auth.ex

@@ -1,4 +1,8 @@
 defmodule MusicLibraryWeb.Auth do
+  use Gettext, backend: MusicLibraryWeb.Gettext
+  import Plug.Conn
+  import Phoenix.Controller, only: [put_flash: 3, redirect: 2]
+
   def correct_login_password?(password) do
     Plug.Crypto.secure_compare(correct_login_password(), password)
   end
@@ -7,4 +11,15 @@ defmodule MusicLibraryWeb.Auth do
     Application.get_env(:music_library, MusicLibraryWeb)
     |> Keyword.fetch!(:login_password)
   end
+
+  def require_logged_in(conn, _opts) do
+    if get_session(conn, :logged_in) do
+      conn
+    else
+      conn
+      |> put_flash(:error, gettext("You must be logged in to access this page"))
+      |> redirect(to: "/login")
+      |> halt()
+    end
+  end
 end