sources: remove canonical_url method.

Refactor source strategies to remove the `canonical_url` method.

`canonical_url` returned the URL that should be used as the source of
the post after upload. Now we simply use `Source::URL#page_url` to
determine the source after upload. If the source is an image URL that is
convertible to a page URL, then the image URL is used as the source. If
the source is an image URL that is not convertible to a page URL, then
the page URL is used as the source.

This simplifies source strategies so that all they have to care about is
implementing the `Source::URL#page_url` and `Sources::Strategies#page_url`
methods, and the preferred source will be chosen for posts automatically.

test/unit/sources/newgrounds_test.rb

@@ -30,9 +30,9 @@ module Sources
         assert_equal([@image_url], @image_2.image_urls)
       end
 
-      should "get the canonical url" do
-        assert_equal(@url, @image_1.canonical_url)
-        assert_equal(@image_url, @image_2.canonical_url)
+      should "get the page url" do
+        assert_equal(@url, @image_1.page_url)
+        assert_equal(@url, @image_2.page_url)
       end
 
       should "download an image" do