Move implementation to own file

Prefactoring, from  TDDAYMI to imperative shell. In the next step we
need the implementation from the functional core in the imperative
shell, to pass in the environment variable.

lib/fun_core/basic_signup_whitelist.ex

@@ -0,0 +1,19 @@
+defmodule FunCore.BasicSignupWhitelist do
+  def normalize(email) do
+    email |> String.trim() |> String.downcase()
+  end
+
+  def addresses_as_list(addresses_str) do
+    addresses_str
+    |> String.split(",")
+    |> Enum.map(&normalize/1)
+  end
+
+  def mail_whitelisted_fun(signups_allowed, email_received) do
+    case signups_allowed do
+      nil -> false
+      "*" -> true
+      list_str -> normalize(email_received) in addresses_as_list(list_str)
+    end
+  end
+end