[FIX] mail: 'chatter app icons', restore design prior to 7a91d876

This commit will restore the chatter App icon design defined for v11
and accidentally removed on 7a91d876.

Apps icons should NOT have a circular shape in order to keep branding
consistency.

addons/mail/static/src/less/client_action.less

@@ -368,6 +368,9 @@
         > img {
             border-radius: 50%;
         }
+        &.o_mail_channel_app > img {
+            border-radius: 2px;
+        }
         .o_mail_user_status {
             .o-position-absolute(@bottom: 0px, @right: 0px);
         }

addons/mail/static/src/xml/client_action.xml

@@ -197,7 +197,7 @@
     <t t-name="mail.chat.ChannelPreview">
         <div t-attf-class="o_mail_channel_preview #{channel.unread_counter ? 'o_channel_unread' : ''}"
              t-att-data-channel_id="channel.id" t-att-data-res_id="channel.res_id" t-att-data-res_model="channel.model">
-            <div class="o_mail_channel_image">
+            <div t-attf-class="o_mail_channel_image #{channel.model? 'o_mail_channel_app' : ''}">
                 <img class="o_mail_channel_image" t-att-src="channel.image_src"/>
                 <i t-if="channel.status === 'online'" class="o_mail_user_status o_user_online fa fa-circle" title="Online"/>
                 <i t-if="channel.status === 'away'" class="o_mail_user_status o_user_idle fa fa-circle" title="Idle"/>

addons/mail/static/src/xml/systray.xml

@@ -60,7 +60,7 @@
         </t>
         <t t-foreach="activities" t-as="activity">
             <div class="o_mail_channel_preview" t-att-data-res_model="activity.model" t-att-data-model_name="activity.name" data-filter='my'>
-                <div class="o_mail_channel_image">
+                <div class="o_mail_channel_image o_mail_channel_app">
                     <img t-att-src="activity.icon"/>
                 </div>
                 <div class="o_channel_info">