Add ability to set minimum crop size

Before this, user can crop the image to any size. The issue with
that is that if you are using paperclip and the cropped image is
smaller than what it needs, it will upsize those selections
resulting in low quality images.
Set inital crop selection to maximum size

So that users only have to downsize their crop area and not upsize
starting from the initial area. This results in a better user
experience.