FK-26 Link up url for thumbnail image

Use the url generated by paperclip to display a thumbnail image in the event list.

Change image settings of paperclip to not mantain image resolution, just take a chunk out
of the image instead to match the required size.

Use the old default image as the missing image for paperclip instead of a default.
Issue #26 Show uploaded image in image trimmer

When an image is uploaded show a preview of the image in the trimmer ui.

Also, the ui now starts hidden, so have it appear before showing the image.