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/source/url/moebooru.rb

@@ -103,7 +103,6 @@ class Source::URL::Moebooru < Source::URL
       filename_prefix = "Konachan.com%20-%20"
     end
 
-    # try to include the post_id so that it's saved for posterity in the canonical_url.
     if post_id.present?
       "https://#{file_host}/image/#{md5}/#{filename_prefix}#{post_id}.#{file_ext}"
     else