social/website_mail_snippet_vertical_resize_base/views/snippets.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- © 2016 Antiun Ingeniería S.L. - Jairo Llopis
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html). -->

<openerp>
<data>

<template id="snippet_hr"
          inherit_id="website_mail.email_designer_snippets">
    <xpath expr="//div[@id='snippet_options']">
        <div
            data-snippet-option-id='vertical_resize'
            data-selector=".vertical_resizable">
            <li>
                <a href="#" class="button js_vertical_resize">
                    Change Height
                </a>
            </li>
        </div>
    </xpath>
</template>

</data>
</openerp>