add multi-select to files

closes: CNVS-12656
closes: CNVS-12724, CNVS-11664

this adds both the ability to multi-select
items (CNVS-12656), and the actions
(like move, delete, download as a zip: CNVS-12724)

test plan:
in new files, click around on the folders/files
in the list.  if you click anywhere other than
a link, button, or input it should toggle whether
that thing is selected (things that are selected
get a blue background). you should be able to
control/shift click, press control+a, control+shift+a
it should work just the mac finder's multi-select

using a screenreader, you should be able to use
the <input type="checkbox"> to toggle selected

be thorough, it should handle all the edge cases ;)

* now that you have some things selected:
* there should be some buttons that appear on the top of the page
* try each:
* download as zip
* move
* delete
* they should all work

Change-Id: I51c2df7d8437d918c2875c293f38e3c60b0122c6
Reviewed-on: https://gerrit.instructure.com/40203
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>