Add configurable prevention of user registration

Should be done appropriately soon.

lib/outlook_web/prevent_registration.ex

@@ -0,0 +1,20 @@
+defmodule Outlook.PreventRegistration do
+
+  import Plug.Conn
+  import Phoenix.Controller
+
+  def prevent_registration(conn, _) do
+    if System.get_env("DISABLE_REGISTRATION") && is_registration_path(conn) do
+      conn
+      |> put_flash(:error, "User Registration is disabled.")
+      |> redirect(to: "/users/log_in")
+      |> halt()
+    else
+      conn
+    end
+  end
+
+  defp is_registration_path(conn) do
+    "/users/register" == current_path(conn, %{})
+  end
+end