Move image resizer to ImageMagick, also reorients the image

This is (I think) the final piece of the new no-cropper Siftr. The client now uploads the entire image straight from the camera, which means the ARIS server needs to resize it (with no crop) and also reorient it according to EXIF data. ImageMagick can do both of these.

The ImageMagick `convert` command-line program can do the reorientation with a simple `-auto-orient` flag but as far as I can tell you have to do it manually like I did here when using the PHP plugin.

I tested this code on my local PHP installation with a few images of different size and EXIF rotation. It requires ImageMagick and the PHP Imagick plugin to be installed on the server -- this should be verified before merging and deploying.