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.

app/logical/sources/strategies/null.rb

@@ -11,10 +11,6 @@ module Sources
         nil
       end
 
-      def canonical_url
-        url
-      end
-
       def artists
         ArtistFinder.find_artists(url)
       end