fix embedded modal content heights, fixes CNVS-15700
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 <bc1e823f4de302371b1d8be26dd82a703439b3d7@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Clare Strong <40d6bb3ed7fef516c73274131d5a837c16de74fd@instructure.com>
Product-Review: Matt Fairbourn <5f353e1c97bb9cc046b9db20beca0b9392f6f1b2@instructure.com>

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