Sharing: Reparent the Email UI to Prevent Broken Layouts

Steps to reproduce:
1. Add the email sharing button to your site.
2. Wrap your theme's `wp_footer()` call in `<div style="position: relative"></div>`
3. Click the email sharing button.

This commit reparents the `sharing_email` div to the body element, and caches a couple jQuery selectors

Props @yoavf 

Original trac ticket: 4811-wpcom