Allow overriding upload path by widget argument

[m-thielen]

I added an optional argument to PageDownWidget named upload_dir. If it is set, IMAGE_UPLOAD_PATH and IMAGE_UPLOAD_UNIQUE are ignored and the provided path is used instead. Like IMAGE_UPLOAD_PATH, upload_dir is inserted between MEDIA_ROOT and the image file name.

If upload_dir is used, the file input element gets a new attribute "data-upload-dir-override" which is added to the POST data by the JavaScript submit handler.

I also added a function that creates a unique file name for the uploaded image. This kind of overlapped with the IMAGE_UPLOAD_UNIQUE feature, but I like my solution better since UUID paths are very bulky. make_unique_path is only called if the upload_dir parameter is used.

If you like it, I can update your README, too.