Default to prior approval values when (re-)reviewing
Default to prior approval values when (re-)reviewing

When leaving a message on a review, default the approval values
to those previously left by the user if they exist.  This way
if the user leaves a follow-up comment or reply, they are less
likely to accidentally clear their vote.

Change-Id: Iab9203e35aef9756057a62fed161fe818dd1d680

stackforge/gertty,openstack/gertty