use max width of 100% on images
use max width of 100% on images

refs CORE-2654

test plan:
 * Hope for the best

Change-Id: I593c778cc311af8086c99240f3d57f87c69dde48
Reviewed-on: https://gerrit.instructure.com/197254
Reviewed-by: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>
Tested-by: Jenkins
QA-Review: Jeremy Neander <20252aad5efdc666f083d82ab63ed0f9f72a043e@instructure.com>
Product-Review: Jeremy Neander <20252aad5efdc666f083d82ab63ed0f9f72a043e@instructure.com>

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