[FIX] mass_mailing: mail template not in english

Mass mailing `mail templates` when selected in a mass mailing are
always used in the english version.

But when another language is involved the edition of mass mailing
`templates` had a number of issue. For example saving the edited english
would only modify the user current language.

These issues are cause because the widget with snippets used for mass
mailing templates edition is not compatible.

Several fixes were proposed, what was choosen finally was to use the
correct widget with the following behavior changes:

- the snippets are not showed anymore in mass mailing templates,
- the current language can be edited and is saved rightly,

The shortcoming this give mass mailing templates can be partly
compensated by:

- editing the template back (if someone want the old behavior),
- duplicating mass mailings which keep the snippets and translations,
- doing a customization to improve the situation

closes #16255
fixes #16165
opw-726746 opw-727232

note: this fix is only needed for 9.0 and saas-11.

addons/mass_mailing/views/email_template.xml

@@ -27,7 +27,7 @@
                         </group>
                         <notebook>
                             <page string="Body">
-                                <field name="body_html" nolabel="1" widget="html_frame" options="{'editor_url': '/mass_mailing/field/email_template'}"/>
+                                <field name="body_html" nolabel="1" widget="html"/>
                                 <field name="attachment_ids" widget="many2many_binary"/>
                             </page>
                         </notebook>