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/models/upload_media_asset.rb

@@ -65,6 +65,20 @@ class UploadMediaAsset < ApplicationRecord
     source_url.starts_with?("file://")
   end
 
+  # The source of the post after upload.
+  def canonical_url
+    return source_url if file_upload?
+
+    # If the image URL is convertible to a page URL, or the page URL couldn't
+    # be found, then use the image URL as the source of the post. Otherwise,
+    # use the page URL.
+    if Source::URL.page_url(source_url).present? || page_url.blank?
+      source_url
+    else
+      page_url
+    end
+  end
+
   def source_strategy
     return nil if source_url.blank?
     Sources::Strategies.find(source_url, page_url)