Thumbs plugin and integration w/the Project page

Let me know if this needs work. The thumbs plugin basically builds thumbnails on the fly so the content of /thumbs is simply a cache folder. You can tell it to crop and you can tell it to upscale if you like (both options I left off).

If you want to move the thumbs directory you can tell the config file where it lives.

Also, for any future thumbs reference: 
https://github.com/bastianallgeier/kirbycms-extensions/tree/master/plugins/thumb