add alt tag to app center images

fixes: PLAT-549

test-plan:
*the thumbnail images for apps in the canvas app center should have an alt tag

Change-Id: I87f1f73a88230fe6a0b1db7555cea3f613ac3618
Reviewed-on: https://gerrit.instructure.com/37092
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
Reviewed-by: Eric Berry <ericb@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>