Change default base_url for zola serve to just /

[orlp]

Currently the base_url for zola serve is 127.0.0.1. This means that viewing pages across devices (such as a mobile phone for checking responsiveness) over the local network is broken by default.

Once #2076 is fixed I would propose we change the default base_url override for zola serve to just /.

Similarly (although this is more controversial due to potential privacy issues) I'd propose we change the default bind address from 127.0.0.1 to 0.0.0.0. With the two above changes zola serve works across the local network by default.

[Keats]

Having a separate base_url for the feed is not a good solution though.