polish for the header in new files.

fixes: CNVS-15375
refs: CNVS-14390

this implements the new multi-select workflow
and makes the header all responsive and nice

test plan:
* select multiple things and delete, move, download them
* the "preview" and "edit status" are buttons are
  their own tickets.
* resize the browser window, the header should resize
  nicely.

Change-Id: I00088e41169c595127ff01d0b672855c6b4c0bc0
Reviewed-on: https://gerrit.instructure.com/41074
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Sterling Cobb <sterling@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>