Check the current password whenever the saved password changes

The profile asked for the current password when the request's password
field was filled in. The check now follows the save itself: the model
checks the challenge whenever the password digest is about to change,
and the profile always hands it the challenge. Editing a name, avatar
or email without a new password still needs none, and scripts that
reset a password, which set no challenge, are unaffected.

Only a string counts as a challenge, and the rate limit counts every
non-empty one, so what's limited is exactly what gets checked.

(cherry picked from commit 4a580302a5)

test/models/user_test.rb

@@ -6,6 +6,14 @@ class UserTest < ActiveSupport::TestCase
     assert users(:david).valid?
   end
 
+  test "a password challenge is checked whenever the password would change" do
+    assert_not User.find(users(:david).id).update(password: "a new secret123456", password_challenge: "")
+    assert_not User.find(users(:david).id).update(password: "a new secret123456", password_challenge: "not my password")
+    assert User.find(users(:david).id).update(name: "Dave", password_challenge: "")
+    assert User.find(users(:david).id).update(password: "a new secret123456", password_challenge: "secret123456")
+    assert User.find(users(:david).id).update(password: "another secret123456")
+  end
+
   test "creating users grants membership to the open rooms" do
     assert_difference -> { Membership.count }, +Rooms::Open.count do
       create_new_user