Address gettext deprecations

Deprecation warning:

	warning: defining a Gettext backend by calling

	    use Gettext, otp_app: ...

	is deprecated. To define a backend, call:

	    use Gettext.Backend, otp_app: :my_app

	Then, instead of importing your backend, call this in your module:

	    use Gettext, backend: MyApp.Gettext

lib/music_library_web.ex

@@ -42,8 +42,9 @@ defmodule MusicLibraryWeb do
         formats: [:html, :json],
         layouts: [html: MusicLibraryWeb.Layouts]
 
+      use Gettext, backend: MusicLibraryWeb.Gettext
+
       import Plug.Conn
-      import MusicLibraryWeb.Gettext
 
       unquote(verified_routes())
     end