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.
Extend crop area to maximum possible width

Crop area now starts at maximum width such that users only have
to downsize until the crop suites their requirement, thus improving
user experience.