Fix remaining 12 multi-line conn shadowing issues

Manually refactor multi-line conn = post/put(conn, ...) patterns
across user_session, user_settings, and user_registration controller
tests. Rename shadowed conn to response using pipeline operator.

Also add plans/multi-line-conn-refactoring.md spec for future
Elixir-based tooling to handle these patterns automatically.

app/test/firehose_web/controllers/user_registration_controller_test.exs

@@ -25,15 +25,16 @@ defmodule FirehoseWeb.UserRegistrationControllerTest do
       Application.put_env(:firehose, :allowed_registration_email, email)
       on_exit(fn -> Application.delete_env(:firehose, :allowed_registration_email) end)
 
-      conn =
-        post(conn, ~p"/users/register", %{
+      response =
+        conn
+        |> post(~p"/users/register", %{
           "user" => valid_user_attributes(email: email)
         })
 
-      refute get_session(conn, :user_token)
-      assert redirected_to(conn) == ~p"/users/log-in"
+      refute get_session(response, :user_token)
+      assert redirected_to(response) == ~p"/users/log-in"
 
-      assert conn.assigns.flash["info"] =~
+      assert response.assigns.flash["info"] =~
                ~r/An email was sent to .*, please access it to confirm your account/
     end
 
@@ -41,12 +42,13 @@ defmodule FirehoseWeb.UserRegistrationControllerTest do
       Application.put_env(:firehose, :allowed_registration_email, "with spaces")
       on_exit(fn -> Application.delete_env(:firehose, :allowed_registration_email) end)
 
-      conn =
-        post(conn, ~p"/users/register", %{
+      response =
+        conn
+        |> post(~p"/users/register", %{
           "user" => %{"email" => "with spaces"}
         })
+        |> html_response(200)
 
-      response = html_response(conn, 200)
       assert response =~ "Register"
       assert response =~ "must have the @ sign and no spaces"
     end