Change footer color in bulk email template

@brianhw real quick one here. Just changed the footer color, addressing https://edx-wiki.atlassian.net/browse/LMS-1185

The change is from #006ba4 to #9FCFE8

I've already changed the color via django-admin on stage, prod, and dev2 (awaiting access to change on stage-edge and edge prod as well). This is just for consistency, as you suggested.