[IMP] gamification: improve email sent when granting a badge

Give some details about the badge: its description, its image, who granted
it, ... Otherwise it is quite complicated to understand the email content.

Thanks to @mart-e who reviewed and tested.

addons/gamification/data/badge.xml

@@ -42,19 +42,42 @@
 
         <record id="email_template_badge_received" model="mail.template">
             <field name="name">Received Badge</field>
+            <field name="subject">New badge ${object.badge_id.name} granted</field>
             <field name="model_id" ref="gamification.model_gamification_badge_user"/>
+            <field name="partner_to">${object.user_id.partner_id.id}</field>
             <field name="lang">${object.user_id.lang}</field>
-            <field name="body_html"><![CDATA[
-        <p>Congratulation, you have received the badge <strong>${object.badge_id.name}</strong> !
-            % if object.sender_id
-                This badge was granted by <strong>${object.sender_id.name}</strong>.
-            % endif
-        </p>
-
-        % if object.comment
-            <p><em>${object.comment}</em></p>
-        % endif
-        ]]></field>
+            <field name="body_html" type="xml">
+<div>
+    <p>Congratulations ${object.user_id.name} !</p>
+    <p>You just received badge <strong>${object.badge_id.name}</strong> !</p>
+</div>
+<div>
+% if object.badge_id.description
+<table cellspacing="0" cellpadding="0" border="0" style="width: 600px; margin-top: 5px;">
+<tbody><tr>
+    <td valign="center">
+        <img src="/web/image/gamification.badge/${object.badge_id.id}/image/80x80" style="padding: 0px; margin: 0px; height: auto; width: 80px;" alt="${user.company_id.name}"/>
+    </td>
+    <td valign="center">
+        <cite>${object.badge_id.description}</cite>
+    </td>
+</tr></tbody>
+</table>
+% endif
+</div>
+<div>
+    <p>
+% if object.sender_id
+        This badge was granted by <strong>${object.sender_id.name}</strong>.
+% endif
+    </p>
+    <p>
+% if object.comment
+        <em>${object.comment}</em>
+% endif
+    </p>
+</div>
+</field>
         </record>
     </data>
 </odoo>

addons/gamification/models/badge.py

@@ -33,18 +33,15 @@ class BadgeUser(models.Model):
         The stats counters are incremented
         :param ids: list(int) of badge users that will receive the badge
         """
-        Template = self.env['mail.template']
-        template_id = self.env.ref('gamification.email_template_badge_received')
+        template = self.env.ref('gamification.email_template_badge_received')
 
         for badge_user in self:
-            # .ids would trigger the "multi mode" which returns a mapping of
-            # res_id to templates
-            template = template_id.get_email_template(badge_user.id)
-            body_html = Template.with_context(template._context).render_template(template.body_html, 'gamification.badge.user', badge_user.id)
-            badge_user.user_id.message_post(
-                body=body_html,
-                subtype='gamification.mt_badge_granted',
-                partner_ids=badge_user.user_id.partner_id.ids
+            self.env['mail.thread'].message_post_with_template(
+                template.id,
+                model=badge_user._name,
+                res_id=badge_user.id,
+                composition_mode='mass_mail',
+                partner_ids=badge_user.user_id.partner_id.ids,
             )
 
         return True