mail_template.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <!-- Form view -->
    <record id="view_email_template_form" model="ir.ui.view">
        <field name="name">view_email_template_form</field>
        <field name="model">mail.template</field>
        <field name="inherit_id" ref="mail.email_template_form" />
        <field name="arch" type="xml">
            <field name="model_id" position="after">
                <field name="is_redner_template" />
            </field>

            <!-- Hide HTML contents, superseded by the body produced by redner. -->
            <xpath expr="//field[@name='body_html']" position="attributes">
                <attribute
                    name="attrs"
                ><![CDATA[{'invisible': [('is_redner_template', '=', True)]}]]></attribute>
            </xpath>

            <!-- Hide the single attachment selector too, not supported with redner. -->
            <xpath expr="//field[@name='attachment_ids']" position="attributes">
                <attribute
                    name="attrs"
                ><![CDATA[{'invisible': [('is_redner_template', '=', True)]}]]></attribute>
            </xpath>

            <!-- Add redner fields instead of the above. -->
            <xpath expr="//field[@name='body_html']" position="before">
                <group
                    string="Redner Template"
                    attrs="{'invisible': [('is_redner_template', '=', False)]}"
                >
                    <field
                        name="redner_tmpl_id"
                        attrs="{'required': [('is_redner_template', '=', True)]}"
                    />
                </group>

                <button
                    confirm="Keywords will be updated. Continue?"
                    name="action_get_substitutions"
                    type="object"
                    string="Get Substitutions from Redner Template"
                    attrs="{'invisible': [('is_redner_template', '=', False)]}"
                />

                <group
                    string="Substitutions"
                    attrs="{'invisible': [('is_redner_template', '=', False)]}"
                >
                    <field name="redner_substitution_ids" nolabel="1" colspan="2">
                        <tree editable="top" default_order="keyword asc">
                            <field name="keyword" />
                            <field name="converter" />
                            <field name="value" />
                        </tree>
                    </field>
                </group>
            </xpath>
        </field>
    </record>
</odoo>