customize "Canvas" in registration confirmation email, fixes CAT-128

add a product_name account setting to allow schools to customize the
registration confirmation email. ostensibly this setting could be used
throughout canvas to allow greater customization, but let's not get ahead
of ourselves

test plan:
1. on your root account, set settings[:product_name] to something
2. /register as a new user
3. confirm that the "set a password" email uses that product name in the
   header and body

Change-Id: I6e0d5aa8ad80c4cda40e2aac1367364293843627
Reviewed-on: https://gerrit.instructure.com/33846
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dave Donahue <ddonahue@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>