Follow-Ups

Tweet: https://twitter.com/lutrov/status/878466898693726208

Follow-ups is not following [the headings hierarchy structure that was improved in WordPress v4.4](https://make.wordpress.org/core/2015/10/28/headings-hierarchy-changes-in-the-admin-screens/) on the `wp-admin/admin.php?page=followup-emails` page`. This is a problem for people using assistive technologies.

Also, input fields are missing appropriate labels. This too is a problem for screen readers and keyboard navigation.

Checked and confirmed this by using the [WP Tota11y](https://wordpress.org/plugins/wp-tota11y/) plugin.