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/tumblr.rb

@@ -15,8 +15,8 @@ module Sources::Strategies
       Danbooru.config.tumblr_consumer_key.present?
     end
 
-    def self.match?(*urls)
-      urls.compact.any? { |url| url.match?(BASE_URL) }
+    def domains
+      ["tumblr.com"]
     end
 
     def site_name