flectra/addons/website_mail_channel/data/mail_template_data.xml

<?xml version="1.0" encoding="utf-8"?>
<flectra>

        <record id="mail_template_list_subscribe" model="mail.template">
            <field name="name">Subscribe to mailing list</field>
            <field name="model_id" ref="mail.model_mail_channel"/>
            <field name="subject">Please confirm subscription to ${object.name}</field>
            <field name="body_html" type="xml">
<p>Hello,</p>
<p>You have requested to be subscribed to the mailing list <strong>${object.name}</strong></p>
<p>To confirm, please visit the following link:</p>
<p><strong><a href="${ctx['token_url']}">${ctx['token_url']}</a></strong></p>
<p>If this was a mistake or you did not requested this action, please ignore this message.</p>
</field>
        </record>

        <record id="mail_template_list_unsubscribe" model="mail.template">
            <field name="name">Unsubscribe to mailing list</field>
            <field name="model_id" ref="mail.model_mail_channel"/>
            <field name="subject">Please confirm unsubscription to ${object.name}</field>
            <field name="body_html" type="xml">
<p>Hello,</p>
<p>You have requested to be unsubscribed to the mailing list <strong>${object.name}</strong></p>
<p>To confirm, please visit the following link:</p>
<p><strong><a href="${ctx['token_url']}">${ctx['token_url']}</a></strong></p>
<p>If this was a mistake or you did not requested this action, please ignore this message.</p>
</field>
        </record>
</flectra>