better handle small browser windows in discussions.
better handle small browser windows in discussions.

fixes CNVS-3720

test plan:
  * open an existing discussion as a user w/ edit
    permissions (e.g. topic author, teacher, or admin);
  * shrink your browser window and verify that the collapse
    and expand entries buttons in the header properly
    collapse below the search input.

Change-Id: Iecfd077fdedc51a973711cdf607cf0d2b7d5f62f
Reviewed-on: https://gerrit.instructure.com/17652
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Joel Hough <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@instructure.com>
QA-Review: Marc LeGendre <d33c80bc45d65303e33ca83108a9952b745af9ef@instructure.com>

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