UX: Create new thumbnail icons to show visibility

[emilyobvs]

It is not clear to users when a work is WPI-only access or Embargoed, unless they scroll down to the bottom of the page. We would like to display the visibility of the work at the top of the page via the thumbnail image.

Derek will work on replacing the current thumbnail image (blank page icon)
-Embargoed works - add "Embargo" text to blank page
-WPI Only works - add "WPI Only" text to blank page

Resources:
https://thenounproject.com/browse/icons/term/restricted-file/

This issue includes further improvements to a closed issue - #462