[FIX] mass_mailing: refactore "Discount Coupon" snippet

* Rename it to "Promo Code"
* Several text changes
* Add a CTA

addons/mass_mailing/views/snippets_themes.xml

@@ -615,8 +615,8 @@
     </div>
 </template>
 
-<template id="s_mail_block_discount2" name="Discount Coupon">
-    <div class="o_mail_block_discount2 mb16">
+<template id="s_mail_block_discount2" name="Promo Code">
+    <div class="o_mail_block_discount2 mb32">
         <div class="o_mail_snippet_general">
             <table align="center" cellspacing="0" cellpadding="0" class="o_mail_table_styles o_mail_h_padding">
                 <tbody>
@@ -626,7 +626,7 @@
                                 <p class="o_mail_display_coupon o_mail_no_margin text-center text-beta" style="font-weight:800;">
                                     $20
                                 </p>
-                                <h3 class="o_mail_no_margin">FROM YOUR NEXT ORDER!</h3>
+                                <h3 class="o_mail_no_margin">OFF YOUR NEXT ORDER!</h3>
                             </div>
                         </td>
                     </tr>
@@ -639,10 +639,15 @@
                                 <small>CODE</small>:   <strong class="o_code h3">45A9E77DGW8455</strong>
                             </p>
                             <p class="text-center">
-                                to receive $20 from your next purchase!
+                                and save $20 on your next order!
                             </p>
                         </td>
                     </tr>
+                    <tr>
+                        <td class="mb16 mt16 text-center">
+                            <a class="btn btn-primary">Use now</a>
+                        </td>
+                    </tr>
                 </tbody>
             </table>
         </div>