Feature request: something to kick to generate thumbnails

[Robinson Tryon]

It could be useful to give the admin a method of asking the server to go generate thumbnails for a bunch of pictures at once.

We could either make this be a "select all images to generate thumbs for," or perhaps just a button that you can click that would go off and work for 30sec or so generating images, and then would return and ask the admin if they wanted to generate more images (that would help prevent PHP from hanging, and would avoid this process "terrorizing" any other processes on the server, especially if we have thousands of images in the database...)