Use app.souma.io instead of modifying /etc/hosts

lmeuser suggested 

> I would suggest [...] changing the DNS entry of app.souma.io (or something like that) to 127.0.0.1 and just using that as the app URL.

https://github.com/ciex/souma/issues/178

This pull request implements that change and it works. This probably breaks offline access, though. The upside is that we don't have to touch `/etc/hosts/ anymore, so theoretically no setup routine is necessary.

What do you think?