Improve matching of username fields in form autofill

 * Match the password field before matching the username field,
   since the password field is easier to match based on the field type.

 * Make username matching more lenient by changing the match key from
   'username' to 'user'
Use <input> placeholder and ARIA labels when determining field purposes

Make use of the 'placeholder' and 'aria-label' attributes when matching
fields against purpose keywords.