Add optional support for post-login redirect

The changes in this pull request enable developers to set the path the module will redirect to, instead of defaulting to /admin/. The redirect path is determined in the following order.

1. A 'next' GET parameter on the link to do_login.
2. The 'LASTFM_AUTH_REDIRECT' setting in the app's settings.
3. Default value of '/admin/'.

I've set the default value for backwards compatibility, so existing projects will not break when they update to this without changing their code, and will continue to function normally.