Update redirect to respect base_url after authentication

[michal-porebski]

Updates the logic for redirecting once a user is authenticated. After the change it respects the base_url setting when generating the redirect URL.

For example, if you run your datasette as follows:
datasette . --setting base_url /tools/datasette/ -p 8009
After successfull authentication you'll be redirected to:
example.com/tools/datasette/
instead of just:
example.com/