Fixed #23461 -- Added EMAIL_TIMEOUT setting

The `timeout` parameter was added to the SMTP backend in [#21271](https://code.djangoproject.com/ticket/21271), but done awkwardly to avoid using a setting. Now that we are fine adding the [setting](https://groups.google.com/d/msg/django-developers/ZWpRbTwTxmo/VJ93xb5V5qoJ), I'm adding it.

Based on [00535e8e6b402cab29ea3dbb7b4cc470f9839678](https://code.djangoproject.com/changeset/00535e8e6b402cab29ea3dbb7b4cc470f9839678/).