fix embedded modal content heights, fixes CNVS-15700

this broke in a recent css/layout refactor

test plan:
1. allow self enrollment on a course, and the join course button
2. as an unenrolled student, click the button
3. the modal should appear and its footer should be at the bottom

Change-Id: I7c33bbae72674b7586b5fb7dda2e61835f7b0754
Reviewed-on: https://gerrit.instructure.com/41471
Reviewed-by: Jeff Belser <jbelser@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>