Use image configs

There are several constants defined in environment.php for image thumbnail sizes, but these are currently ignored in favour of a hard-coded solution. This pull request removes the hard-coding and uses constants defined in environment.php to specify image sizes. The constants are now specified as arrays (see http://stackoverflow.com/questions/1290318/php-constants-containing-arrays). The values contain the current hard-coded values, so this code should not change current EoL behaviour.

Since the constants are named, e.g. "LARGE", "MEDIUM", "SMALL", this code also creates a symbolic link to the created thumbnails under that name, so that thumbnails created as e.g. 29578_88_88.jpg are also symlinked as 29578_CROPPED_SMALL.jpg, and 29578_580_360.jpg is linked to by 29578_SCALED_LARGE.jpg. Please comment as to whether symlinks are preferable to hard links here.