[FIX] mass_mailing: open links in parent while in backend

Before this commit, links that had no target location opened in _self, which is the iframe within which they are embedded.
After this commit, thos links are now opened in _parent to simulate closer the beahavior with an actual e-mail

addons/mass_mailing/views/editor_field_html.xml

@@ -12,6 +12,7 @@
                 <t t-call-assets="mass_mailing.assets_mail_themes"/>
                 <script type="text/javascript" src="/mass_mailing/static/src/js/mass_mailing_editor.js"></script>
             </t>
+            <base target="_parent"/>
         </t>
         <t t-set="head" t-value="head_mm_field_text_html_inline + (head or '')"/>