Refactor FilePreview Component

Fixes a bug with closing the modal when
multiple files are selected; cleans up
the FilePreview component for New Files;
breaks up FilePreview into multiple
components.

Change-Id: I9d4796c57f224a30e601588a2e366781c58c8449
Reviewed-on: https://gerrit.instructure.com/42439
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>
Tested-by: Ryan Shaw <ryan@instructure.com>