Add admin command to configure site url

[atodorov]

This is how the confirmation email looks like when Site URL isn't configured:

===
Welcome ....,
thank you for signing up for an 127.0.0.1:8000 account!

To activate your account, click this link:
https://127.0.0.1:8000/accounts/confirm/
b2c1f9db7ad1ce5a7191cdd20849c8ee8ce3f76367b0d5ae6fa0313ba967f86d/
===

I'm still not in favor of exposing this via the web and letting everyone do it (before there is a 1st account) but a django command should be easy to implement.

[atodorov]

Duplicate of #971 which has more discussion.