Don't use Current.account for search records

It doesn't actually work, and even if we could make it work reliably
we are better off if the records always know to go to the right shard.

It does make the interface a bit more complicated as we need to select
the right shard class with `for(account_id)`.

lib/tasks/search.rake

@@ -12,16 +12,10 @@ namespace :search do
     end
 
     puts "Reindexing cards..."
-    Card.find_each do |card|
-      Current.account = card.account
-      card.reindex
-    end
+    Card.includes(:rich_text_description).find_each(&:reindex)
 
     puts "Reindexing comments..."
-    Comment.find_each do |comment|
-      Current.account = comment.account
-      comment.reindex
-    end
+    Comment.includes(:rich_text_body, :card).find_each(&:reindex)
 
     puts "Done! Reindexed #{Card.count} cards and #{Comment.count} comments."
   end