social/mail_activity_done/views/templates.xml

<?xml version="1.0" encoding="UTF-8"?>
<odoo>
    <!--
        Copyright 2018 Eficent <http://www.eficent.com>
        License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl.html).
    -->
    <template id="assets_backend" name="mail_activity_done assets" inherit_id="web.assets_backend">
        <xpath expr="." position="inside">
            <script type="text/javascript" src="/mail_activity_done/static/src/js/mail_activity.js"/>
        </xpath>
    </template>
</odoo>