R1:d75007cf42d1

Validate logins, and simplify email password resets

Summary:
- There are some recent reports of login issues, see T755 and T754. I'm not
really sure what's going on, but this is an attempt at getting some more
information.
- When we login a user by setting 'phusr' and 'phsid', send them to
/login/validate/ to validate that the cookies actually got set.
- Do email password resets in two steps: first, log the user in. Redirect them
through validate, then give them the option to reset their password.