Give the changing browser a new session rather than keeping its own

Keeping the session the password was changed from also kept any copy of
its cookie working, and a copied cookie is one of the ways someone else
gets into an account. Every session ends now, and the profile page
signs the browser that changed the password back in with a new one.

From #371.

(cherry picked from commit b01bb7a0a7)

app/controllers/users/profiles_controller.rb

@@ -8,6 +8,8 @@ class Users::ProfilesController < ApplicationController
 
   def update
     @user.update user_params
+    start_new_session_for @user if @user.saved_change_to_password_digest?
+
     redirect_to user_profile_url, notice: update_notice
   end