[FIX] mail: assign correct subtype to "general" demo messages

Some demo messages have a grey background because they have the subtype_id set to
NULL.
The subtype should be `mail.mt_comment`, like other messages inside the general channel.

task-2278438

closes odoo/odoo#54261

X-original-commit: 9be1079cc62699d119b9c2acc6282bd72f71f9af
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/data/mail_channel_demo.xml

@@ -65,6 +65,7 @@
             <field name="message_type">comment</field>
             <field name="author_id" ref="base.partner_demo"/>
             <field name="date" eval="(DateTime.today() - timedelta(minutes=22)).strftime('%Y-%m-%d %H:%M')"/>
+            <field name="subtype_id" ref="mail.mt_comment"/>
         </record>
         <record id="mail_message_channel_whole_2" model="mail.message">
             <field name="model">mail.channel</field>
@@ -75,6 +76,7 @@
             <field name="message_type">comment</field>
             <field name="author_id" ref="base.partner_demo"/>
             <field name="date" eval="(DateTime.today() - timedelta(hours=1)).strftime('%Y-%m-%d %H:%M')"/>
+            <field name="subtype_id" ref="mail.mt_comment"/>
         </record>
         <record id="mail_message_channel_whole_2_1" model="mail.message">
             <field name="model">mail.channel</field>
@@ -85,6 +87,7 @@
             <field name="message_type">comment</field>
             <field name="author_id" ref="base.partner_root"/>
             <field name="date" eval="(DateTime.today() - timedelta(minutes=34)).strftime('%Y-%m-%d %H:%M')"/>
+            <field name="subtype_id" ref="mail.mt_comment"/>
         </record>
         <record id="mail_message_channel_whole_2_2" model="mail.message">
             <field name="model">mail.channel</field>
@@ -95,6 +98,7 @@
             <field name="message_type">comment</field>
             <field name="author_id" ref="base.partner_demo"/>
             <field name="date" eval="(DateTime.today() - timedelta(minutes=22)).strftime('%Y-%m-%d %H:%M')"/>
+            <field name="subtype_id" ref="mail.mt_comment"/>
         </record>
 
         <!-- Board messages -->
@@ -161,5 +165,3 @@
 
     </data>
 </odoo>
-
-