[FIX] mail: fix template preview wizard view

Before this commit, when on a mail.template form, click on the Preview smart button.
A wizard popped up with two buttons, as the default for form views.
First this made no sense since the wizard doesn't represent a model that can be modified
Second, the button discard was unable to close the wizard.

After this commit, only a discard button is present which does its job.

addons/mail/wizard/email_template_preview_view.xml

@@ -28,6 +28,9 @@
                     <field name="body_html" widget="html" readonly="1"
                             nolabel="1" options='{"safe": True}'/>
                     <field name="attachment_ids" widget="many2many_binary" readonly="1"/>
+                    <footer>
+                        <button string="Discard" class="btn-default" special="cancel"/>
+                    </footer>
                 </form>
             </field>
         </record>