[ADD] mass_mailing: add background image to s_cover

The `s_cover` snippet was designed to have text over it but this was not
supported in `mass_mailing` because of a limitation of Outlook on
Windows so we replaced it with just an image with no text. Now that we
support text over images thanks to the conversion process, we can offer
that possibility for the `s_cover` snippet in `mass_mailing` as well.
Note that buttons over images are not supported so they were not added
to the snippet like they are in `website`.

task-2856858

Part-of: odoo/odoo#91314

addons/mass_mailing/views/snippets/s_cover.xml

@@ -2,7 +2,14 @@
 <odoo>
     <template id="s_cover" name="Cover">
         <div class="s_cover o_mail_snippet_general" data-snippet="s_cover">
-            <img src="/web/image/mass_mailing.s_cover_default_image" alt="Cover image" class="img-fluid w-100 mx-auto"/>
+            <div class="container">
+                <div class="row">
+                    <div class="col-lg-12 oe_img_bg  pt56 pb48" style="background-image: url('/web/image/mass_mailing.s_cover_default_image');">
+                        <h1 style="text-align: center;"><font style="font-size: 62px; font-weight: bold;">Catchy Headline</font></h1>
+                        <p class="lead" style="text-align: center;">Write one or two paragraphs describing your product, services or a specific feature.<br/> To be successful your content needs to be useful to your readers.</p>
+                    </div>
+                </div>
+            </div>
         </div>
     </template>
 </odoo>

addons/mass_mailing/views/snippets_themes.xml

@@ -484,9 +484,9 @@
             data-color-prefix="bg-"/>
     </div>
 
-    <!-- Allow changing background images in Masonry -->
+    <!-- Allow changing background images in Masonry and Cover -->
     <t t-call="web_editor.snippet_options_background_options">
-        <t t-set="selector" t-value="'.s_masonry_block .row > div'"/>
+        <t t-set="selector" t-value="'.s_masonry_block .row > div, .s_cover .oe_img_bg'"/>
         <t t-set="with_images" t-value="True"/>
     </t>