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.rb

@@ -16,8 +16,8 @@ module Sources
     end
 
     def self.find(url, referer=nil, default: Strategies::Null)
-      strategy = all.detect { |strategy| strategy.match?(url, referer) } || default
-      strategy&.new(url, referer)
+      strategy = all.map { |strategy| strategy.new(url, referer) }.detect(&:match?)
+      strategy || default.new(url, referer)
     end
 
     def self.canonical(url, referer)