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.

app/logical/source/url/twitter.rb

@@ -29,7 +29,8 @@ class Source::URL::Twitter < Source::URL
   attr_reader :status_id, :twitter_username, :user_id
 
   def self.match?(url)
-    url.host.in?(%w[twitter.com mobile.twitter.com pic.twitter.com pbs.twimg.com video.twimg.com t.co])
+    return false if Source::URL::TwitPic.match?(url) # TwitPic uses https://o.twimg.com/ URLs
+    url.domain.in?(%w[twitter.com twimg.com t.co])
   end
 
   def parse