There are a few changes here to make the Emails admin page more consistent with the rest of the admin UI.
- The header and navigation menu have been updated.
- The sidebar now stays highlighted when visiting the email admin sub-pages.
- Moved the Template editor from /admin/customize/email_templates to /admin/email/templates, so it fit as a sub-page.
- Removed the link to the Template editor from the Customize section of the old top menu, since it's accessible from the Emails section, instead.
