Achievements Admin Page With Thumbnails

I have added the code necessary to show thumbnails in the admin page. Below is a preview of what it might look like. Disregard the Category columns as that is not part of this functionality.

The thumbnail header is not sortable at this time -- I didn't think sorting by image made sense.

Added comments in two places where changes can be made, theoretically in a Settings page if a UI were to be added in the future. One for column placement (default is 2nd), other for header title (default is Badge).