sources: fix sources sometimes choosing wrong strategy (fix #3968)

Fix sources choosing the wrong strategy when the referer belongs to a
different site (for example, when uploading a twitter post with a pixiv
referer).

* Fix `match?` to only consider the main url, not the referer.

* Change `match?` to match against a list of domains given by the `domains` method.

* Change `match?` to an instance method.

app/logical/sources/strategies/stash.rb

@@ -15,8 +15,12 @@ module Sources
     class Stash < DeviantArt
       STASH = %r{\Ahttps?://sta\.sh/(?<post_id>[0-9a-zA-Z]+)}i
 
-      def self.match?(*urls)
-        urls.compact.any? { |x| x =~ STASH }
+      def domains
+        ["deviantart.net", "sta.sh"]
+      end
+
+      def match?
+        parsed_urls.map(&:domain).any?("sta.sh")
       end
 
       def site_name