posts: fix exception when post has source like 'https://www.twitter.com/username'.

`twitter.com` sources worked but `www.twitter.com` didn't.

Also match the URL by class instead of by site name to ensure we match
the expected class.

test/unit/sources/twitter_test.rb

@@ -302,6 +302,7 @@ module Sources
         assert_equal(source2, Sources::Strategies.normalize_source(source2))
         assert_equal(source2, Sources::Strategies.normalize_source(source3))
         assert_equal(source2, Sources::Strategies.normalize_source(source4))
+        assert_equal("https://www.twitter.com/irt_5433", Sources::Strategies.normalize_source("https://www.twitter.com/irt_5433"))
       end
     end
   end