sources: add Gelbooru support.

Add support for uploading posts from Gelbooru. Note that the translated
tags will include both the Gelbooru tags and the tags from the Gelbooru
post's source. The commentary and artist information will also be taken
from the Gelbooru post's source. The source of the Danbooru post however
will be left as the Gelbooru post itself, not as the Gelbooru post's source.

app/logical/source/url/gelbooru.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+class Source::URL::Gelbooru < Source::URL
+  attr_reader :post_id, :md5, :full_image_url
+
+  def self.match?(url)
+    url.domain.in?(%w[gelbooru.com])
+  end
+
+  def parse
+    case [domain, *path_segments]
+
+    # https://gelbooru.com/index.php?page=post&s=view&id=7798045
+    # https://www.gelbooru.com/index.php?page=post&s=view&id=7798045
+    in "gelbooru.com", "index.php" if params[:page] == "post" && params[:s] == "view" && params[:id].present?
+      @post_id = params[:id].to_i
+
+    # https://gelbooru.com/index.php?page=post&s=list&md5=99d9977d6c3aa185083a2da22bd8acfb
+    in "gelbooru.com", "index.php" if params[:page] == "post" && params[:s] == "list" && params[:md5].present?
+      @md5 = params[:md5]
+
+    # https://gelbooru.com/index.php?page=dapi&s=post&q=index&id=7798045&json=1
+    in "gelbooru.com", "index.php" if params[:page] == "dapi" && params[:q] == "index" && params[:id].present?
+      @post_id = params[:id].to_i
+
+    # https://gelbooru.com//images/ee/5c/ee5c9a69db9602c95debdb9b98fb3e3e.jpeg
+    # http://simg.gelbooru.com//images/2003/edd1d2b3881cf70c3acf540780507531.png
+    # https://simg3.gelbooru.com//samples/0b/3a/sample_0b3ae5e225072b8e391c827cb470d29c.jpg
+    # https://video-cdn3.gelbooru.com/images/62/95/6295154d082f04009160261b90e7176e.mp4
+    # https://img2.gelbooru.com//images/a9/64/a96478bbf9bc3f0584f2b5ddf56025fa.webm
+    # https://gelbooru.com/thumbnails/08/06/thumbnail_08066c138e7e138a47489a0934c29156.jpg
+    in "gelbooru.com", ("images" | "samples" | "thumbnails"), h1, h2, /\A(?:\w+_)?(\h{32})\.(jpeg|jpg|png|gif|mp4|webm)\z/i
+      @md5 = $1
+      @full_image_url = "https://#{host}/images/#{h1}/#{h2}/#{md5}.#{file_ext}"
+
+    else
+      nil
+    end
+  end
+
+  def image_url?
+    full_image_url.present?
+  end
+
+  def page_url
+    if post_id.present?
+      "https://gelbooru.com/index.php?page=post&s=view&id=#{post_id}"
+    elsif md5.present?
+      "https://gelbooru.com/index.php?page=post&s=list&md5=#{md5}"
+    end
+  end
+
+  def api_url
+    "https://gelbooru.com/index.php?page=dapi&s=post&q=index&id=#{post_id}&json=1" if post_id.present?
+  end
+end

app/logical/source/url/null.rb

@@ -131,15 +131,6 @@ class Source::URL::Null < Source::URL
       @work_id = $1
       @page_url = "https://www.facebook.com/photo.php?fbid=#{work_id}"
 
-    # https://gelbooru.com//images/ee/5c/ee5c9a69db9602c95debdb9b98fb3e3e.jpeg
-    # http://simg.gelbooru.com//images/2003/edd1d2b3881cf70c3acf540780507531.png
-    # https://simg3.gelbooru.com//samples/0b/3a/sample_0b3ae5e225072b8e391c827cb470d29c.jpg
-    # https://video-cdn3.gelbooru.com/images/62/95/6295154d082f04009160261b90e7176e.mp4
-    # https://img2.gelbooru.com//images/a9/64/a96478bbf9bc3f0584f2b5ddf56025fa.webm
-    in _, "gelbooru.com", ("images" | "samples"), *subdirs, /^(?:sample_)?(\h{32})\.(jpeg|jpg|png|gif|mp4|webm)$/
-      @md5 = $1
-      @page_url = "https://gelbooru.com/index.php?page=post&s=list&tags=md5:#{@md5}"
-
     # https://a.hitomi.la/galleries/907838/1.png
     # https://0a.hitomi.la/galleries/1169701/23.png
     # https://aa.hitomi.la/galleries/990722/003_01_002.jpg