use absolute url for avatar in notifications

fixes CNVS-14251

test plan
- cause a notification with an avatar to be sent (discussion,
 conversation, or submission comment)
- ensure that the avatar image src attribute is an absolute url

Change-Id: I6992de1f4b83ecfdf838725a42e09feb1ca5e1c9
Reviewed-on: https://gerrit.instructure.com/37729
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>