add empty state messages for files

fixes: #CNVS-12665

test plan:
* browse to an empty folder, it should look like
  https://instructure.atlassian.net/secure/attachment/17936/empty-state.png
* type a search query that doesn't match anything.
  it should look like http://cl.ly/image/1W2F0u1h2b0G

Change-Id: Ia6a71b18bae3cd3584d83207ce1056f31410d988
Reviewed-on: https://gerrit.instructure.com/38920
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>