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)

app/models/user.rb

@@ -18,7 +18,7 @@ class User < ApplicationRecord
   enum :status, %i[ active deactivated banned ], default: :active
 
   has_secure_password validations: false
-  validate :password_challenge_matches, unless: -> { password_challenge.nil? }
+  validate :password_challenge_matches, if: :will_save_change_to_password_digest?, unless: -> { password_challenge.nil? }
 
   after_create_commit :grant_membership_to_open_rooms
   after_update :sign_out_all_sessions, if: :saved_change_to_password_digest?
@@ -52,7 +52,8 @@ class User < ApplicationRecord
   end
 
   private
-    # The profile sets password_challenge whenever it changes the password; scripts that reset one don't.
+    # The profile always sets password_challenge, and it's checked whenever the save would change the password; scripts
+    # that reset one don't set it.
     def password_challenge_matches
       unless password_digest_was.present? && BCrypt::Password.new(password_digest_was).is_password?(password_challenge)
         errors.add :password_challenge, :invalid