post tooltips: add header bar with more post info.

Add a header bar containing the following information:

* Uploader + top tagger + upload date
* Score, favorite count, comment count
* Rating
* Source (Pixiv, Twitter, etc)
* Image dimensions

Also list series pools with tags.

app/models/post.rb

@@ -509,6 +509,15 @@ class Post < ApplicationRecord
         source
       end
     end
+
+    def source_domain
+      return "" unless source =~ %r!\Ahttps?://!i
+
+      url = Addressable::URI.parse(normalized_source)
+      url.domain
+    rescue
+      ""
+    end
   end
 
   module TagMethods