IE Compatibility for New Files UI

Fixes: CNVS-16385, CNVS-16776

Fixes Flexbox-related layout bugs in the files list
table header columns and the File Preview overlay.
Please see JIRA for screenshots of the bugs.

Test plan:
- Test in both IE10 and 11, as well as one additional
  browser to verify nothing has broken as a result of
  the IE fixes
- Make sure you have Better File Browsing enabled for
  your test course (under Settings > Feature Options)
- From the main course page, click on Files
- Add a file to your course, if you haven't already
- Click the eyeball icon to view the file in the
  File Preview overlay
- While doing the above, verify that the bugs in
  both JIRA tickets are addressed in both IE10 and IE11.

Change-Id: Ibf7467c2f6c7f9720feda9bd545b316b6559eee6
Reviewed-on: https://gerrit.instructure.com/44356
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
Product-Review: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>