sources: factor out Source::URL::Pixiv.

* Drop support for preview_urls. This means that IQDB lookups may be
  slower, especially for ugoiras, since we have to download the full
  ugoira now. However, ugoira lookups should produce better results,
  since the ugoira thumbnail chosen by Pixiv wasn't necessarily the same
  as the thumbnail chosen by Danbooru.

* Drop support for uploading single manga pages:

    http://www.pixiv.net/member_illust.php?mode=manga_big&illust_id=18557054&page=2

  Previously uploading an URL like this would only upload a single image
  out of a multi-image work. Now it will upload all images in the work.
  Pixiv no longer supports URLs like this, so we don't either.

* Add support for parsing URLs like this:

    https://i.pximg.net/c/360x360_70/custom-thumb/img/2022/03/08/00/00/56/96755248_p0_custom1200.jpg

  Apparently artists can choose a custom thumbnail now (not like anyone
  will try to upload one though).

test/unit/sources/pixiv_test.rb

@@ -8,7 +8,7 @@ module Sources
 
     def assert_illust_id(illust_id, url)
       site = Sources::Strategies.find(url)
-      assert_equal(illust_id, site.illust_id)
+      assert_equal(illust_id, site.illust_id.to_i)
       assert_nothing_raised { site.to_h }
     end
 
@@ -43,10 +43,6 @@ module Sources
           assert_equal("https://i.pximg.net/img-zip-ugoira/img/2017/04/04/08/57/38/62247364_ugoira1920x1080.zip", @site.file_url)
         end
 
-        should "get the preview url" do
-          assert_equal("https://i.pximg.net/c/240x240/img-master/img/2017/04/04/08/57/38/62247364_master1200.jpg", @site.preview_url)
-        end
-
         should "capture the frame data" do
           media_file = @site.download_file!
 
@@ -103,10 +99,6 @@ module Sources
           assert_equal("https://i.pximg.net/img-original/img/2017/11/21/05/12/37/65981735_p0.jpg", @site.image_url)
         end
 
-        should "get the preview size image url" do
-          assert_equal("https://i.pximg.net/c/240x240/img-master/img/2017/11/21/05/12/37/65981735_p0_master1200.jpg", @site.preview_url)
-        end
-
         should "get the page count" do
           assert_equal(1, @site.image_urls.size)
         end