End sign-in links made before a password change

A transfer link signed only the user id, so one copied before the
password changed still started a session for up to four hours after.
Bind it to the password salt with generates_token_for, so a new
password ends those links too. Links made before this upgrade stop
working; they last four hours at most.

From #371.

(cherry picked from commit 160ba295b5)

test/models/user/transferable_test.rb

@@ -0,0 +1,40 @@
+require "test_helper"
+
+class User::TransferableTest < ActiveSupport::TestCase
+  test "a transfer id finds its user" do
+    assert_equal users(:david), User.find_by_transfer_id(users(:david).transfer_id)
+  end
+
+  test "a transfer id expires" do
+    transfer_id = users(:david).transfer_id
+
+    travel User::TRANSFER_LINK_EXPIRY_DURATION + 1.second
+
+    assert_nil User.find_by_transfer_id(transfer_id)
+  end
+
+  test "a transfer id made before a password change stops working" do
+    transfer_id = users(:david).transfer_id
+
+    users(:david).update!(password: "a new secret123456")
+
+    assert_nil User.find_by_transfer_id(transfer_id)
+    assert_equal users(:david), User.find_by_transfer_id(users(:david).transfer_id)
+  end
+
+  test "other changes keep a transfer id working" do
+    transfer_id = users(:david).transfer_id
+
+    users(:david).update!(name: "Dave", email_address: "dave@37signals.com")
+
+    assert_equal users(:david), User.find_by_transfer_id(transfer_id)
+  end
+
+  test "a transfer id finds only users in scope" do
+    transfer_id = users(:david).transfer_id
+
+    users(:david).deactivate
+
+    assert_nil User.active.find_by_transfer_id(transfer_id)
+  end
+end