Fix Mix.env/0 unavailable in production release

Author: ericmj

config/config.exs

@@ -1,6 +1,7 @@
 import Config
 
 config :hexdocs,
+  scheme: "http",
   port: "4002",
   hexpm_url: "http://localhost:4000",
   hexpm_secret: "2cd6d09334d4b00a2be4d532342b799b",

config/prod.exs

@@ -1,6 +1,7 @@
 import Config
 
 config :hexdocs,
+  scheme: "https",
   hexpm_impl: Hexdocs.Hexpm.Impl,
   store_impl: Hexdocs.Store.Impl,
   cdn_impl: Hexdocs.CDN.Fastly,

lib/hexdocs/plug.ex

@@ -102,7 +102,7 @@ defmodule Hexdocs.Plug do
   end
 
   defp build_oauth_redirect_uri(_conn, organization) do
-    scheme = if Mix.env() == :prod, do: "https", else: "http"
+    scheme = Application.get_env(:hexdocs, :scheme)
     host = Application.get_env(:hexdocs, :host)
     "#{scheme}://#{organization}.#{host}/oauth/callback"
   end