Join with website_request_logs to get message_text for headers as a new column in user_experiment_info

Summary: NOTE: It seems there are some special characters that should be escaped, working on that now, but the raw message_text is in the table now

Test Plan: Succesfully ran new scripts_combined.q in an interactive hive cluster, user_experiment_info now has a message_text field, and growth mindset header experiments now have values in that field

Reviewers: jascha, sitanchen

Reviewed By: jascha

Differential Revision: http://phabricator.khanacademy.org/D3118