Avoid reloading the entire app when authenticating

[Fab1en]

The window.location.replace('/'); line in admin/src/authProvider.js reloads the entire app when authentication is done. This is only necessary because the localStorage.getItem('token') is called once during Admin component startup and not every time a request is done. By moving this call inside fetchHydra, the token refresh is taken into account immediately, without needing a full app refresh.

[dunglas]

ping @mauchede