Achievement 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).

Thumbnail size is set to 39px to fit in current row heigh, but can be made larger without any problem.