Extract host from url.

Parse the url of phoenix endpoint.

When deploying Phoenix application behind a proxy, it is common to specify the :url option of Phoenix endpoint like this:

config :demo_web, DemoWeb.Endpoint,
  url: [scheme: "https", host: "example.com", port: 443, path: "/"],
  check_origin: ["https://example.com/"],
  # ...

As you can see, the config is verbose because :url and :check_origin are sharing the same pieces of data.

In order to remove this kind of verbose, we can write something like:

import Retro.ConfigHelper

base_url = "https://example.com/"

config :demo_web, DemoWeb.Endpoint,
  url: parse_phoenix_endpoint_url(base_url),
  check_origin: [base_url],
  # ...

Much better.