Refuse a session for a password that changed after it was checked

A password change ends every session, but a sign-in or sign-in link checked
against the old password just before the change could still save its session
just after, and that session outlived the change.

The session now checks, as it's saved, that the user's password is still the
one the request loaded. The save's transaction holds SQLite's write lock from
the start, so a change commits either before the check, which fails it, or
after the insert, and ends this session with the others. Sessions start
through the user's association so the check sees the user that was
authenticated, and a refused sign-in is answered like a wrong password.

(cherry picked from commit 19319a1cff)

app/controllers/concerns/authentication.rb

@@ -57,7 +57,7 @@ module Authentication
     end
 
     def start_new_session_for(user)
-      user.sessions.start!(user_agent: request.user_agent, ip_address: request.remote_ip).tap do |session|
+      user.sessions.create!(user_agent: request.user_agent, ip_address: request.remote_ip).tap do |session|
         authenticated_as session
       end
     end