Strip <title>, <script> and <style> tags from HTML emails.

Summary:  Fixes T162. The code which strips HTML preserves the contents of the tags but it doesn't make sense to keep inline Javascript and CSS.

Test Plan: Ran a full sync. Checked results for HTML emails.

Reviewers: spang

Reviewed By: spang

Maniphest Tasks: T162

Differential Revision: https://review.inboxapp.com/D281