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_settings_controller_test.exs

@@ -71,28 +71,30 @@ defmodule FirehoseWeb.UserSettingsControllerTest do
   describe "PUT /users/settings (change email form)" do
     @tag :capture_log
     test "updates the user email", %{conn: conn, user: user} do
-      conn =
-        put(conn, ~p"/users/settings", %{
+      response =
+        conn
+        |> put(~p"/users/settings", %{
           "action" => "update_email",
           "user" => %{"email" => unique_user_email()}
         })
 
-      assert redirected_to(conn) == ~p"/users/settings"
+      assert redirected_to(response) == ~p"/users/settings"
 
-      assert Phoenix.Flash.get(conn.assigns.flash, :info) =~
+      assert Phoenix.Flash.get(response.assigns.flash, :info) =~
                "A link to confirm your email"
 
       assert Accounts.get_user_by_email(user.email)
     end
 
     test "does not update email on invalid data", %{conn: conn} do
-      conn =
-        put(conn, ~p"/users/settings", %{
+      response =
+        conn
+        |> put(~p"/users/settings", %{
           "action" => "update_email",
           "user" => %{"email" => "with spaces"}
         })
+        |> html_response(200)
 
-      response = html_response(conn, 200)
       assert response =~ "Settings"
       assert response =~ "must have the @ sign and no spaces"
     end