#basename
#filename
#file_ext
Add `#basename`, `#filename`, and `#file_ext` utility methods to Danbooru::URL and change a few places to use them. Simplifies parsing filenames in source URLs in various places.
Also fix it so that the image URL is set as the source for Newgrounds posts, not the page URL. It's possible to generate the page URL from the image URL (except for images after the first in multi-image posts). * Page: https://www.newgrounds.com/art/view/natthelich/weaver * Image: https://art.ngfiles.com/images/1520000/1520217_natthelich_weaver.jpg?f1606365031