add image to course invitation email

add image to course invitation email. this will be a blue 'get started' button in the email to get the users attention.

fixes CNVS-14292

test plan:
1. Go to Course > People
2. Add a user to course via + People button
3. After adding user, get user id and access /users/:id/messages
4. Click HTML tab and verify 'get started' button

Change-Id: I00ee58688e86e5421ba1f0b486c563f1301be651
Reviewed-on: https://gerrit.instructure.com/39398
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>