Introduce new welcome page for new projects

As requested by David in 23233.

railties/test/application/routing_test.rb

@@ -42,8 +42,7 @@ def teardown
     test "root takes precedence over internal welcome controller" do
       app("development")
 
-      get '/'
-      assert_match %r{<h1>Getting started</h1>} , last_response.body
+      assert_welcome get('/')
 
       controller :foo, <<-RUBY
         class FooController < ApplicationController

railties/test/isolation/abstract_unit.rb

@@ -74,10 +74,12 @@ def get(path)
     end
 
     def assert_welcome(resp)
+      resp = Array(resp)
+
       assert_equal 200, resp[0]
       assert_match 'text/html', resp[1]["Content-Type"]
       assert_match 'charset=utf-8', resp[1]["Content-Type"]
-      assert extract_body(resp).match(/Welcome aboard/)
+      assert extract_body(resp).match(/Yay! You.*re on Rails!/)
     end
 
     def assert_success(resp)