Don't load ES on checkout or login pages (peace of mind thing)

Only a minor change in response to http://www.reddit.com/r/Steam/comments/1lwyg9/quick_question_dont_upvote_can_enhanced_steam_see/

ES doesn't do anything on these pages and they're known to contain sensitive information, so for the peace-of-mind of users I suggest we just don't allow the extension to load on them.