[FIX] mass_mailing: display Unsubscribe link in mail preview

Steps to reproduce:

  - Install `mass_mailing` module
  - Go to mass_mailing and create a new mailing
  - Add a `from scratch` snippet
  - Add a `footer center` block
  - Save and click on `Mail Debug` tab

Issue:

  The `Unsubscribe` link is not displayed in the preview.

Cause:

  When converting the Bootstrap rows to table, we convert only the row's
  childrens that have a `col` class:
  https://github.com/odoo/odoo/blob/4ba61316a4dd212d6a572e3f9fcc250815b912f0/addons/web_editor/static/src/js/backend/convert_inline.js#L12
  https://github.com/odoo/odoo/blob/4ba61316a4dd212d6a572e3f9fcc250815b912f0/addons/web_editor/static/src/js/backend/convert_inline.js#L186

Solution:

  Add a `col-lg` CSS class on the first children of the row.

opw-3104011

closes odoo/odoo#112697

X-original-commit: 0a3d7ffb5132d7a68fbd75669903bcb0af25b584
Signed-off-by: Nasreddin Boulif (bon) <bon@odoo.com>

addons/mass_mailing/views/snippets_themes.xml

@@ -198,7 +198,7 @@
                 </div>
             </div>
             <div class="row">
-                <div class="o_mail_footer_links">
+                <div class="col-lg o_mail_footer_links">
                     <a role="button" href="/unsubscribe_from_list" class="btn btn-link">Unsubscribe</a>
                 </div>
             </div>