Bug 30223: Move book cover image upload JS to a separate file

Bug 30223: Move book cover image upload JS to a separate file

This patch moves most of the JavaScript from upload-images.tt into a
separate file, upload-images.js. Functionality should be unchanged.

To test, apply the patch and go to Tools  -> Upload local cover image.

Test all the things:
 - Drag-and-drop upload
 - Click-to-browse upload;
 - Deleting existing images
 - Uploading a zip file

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>