[FIX] mail: adapt discuss mobile from cp refactoring

Since a recent ControlPanel/Search view refactoring [1],
all client actions must be adapted [2]. Discuss was updated,
but not discuss mobile, which this commit fixes.

[1] https://github.com/odoo/odoo/commit/4cd379cf6ecffa00e0085a9833228128b484d48a
[2] https://github.com/odoo/odoo/commit/b1ef0001f61d29fc3d3be384422c0fb35a09806f

Task-ID 1918722

closes odoo/odoo#29601

addons/mail/static/src/js/discuss_mobile.js

@@ -16,7 +16,7 @@ if (!config.device.isMobile) {
  * Overrides Discuss module in mobile
  */
 Discuss.include({
-    template: 'mail.discuss_mobile',
+    contentTemplate: 'mail.discuss_mobile',
     events: _.extend(Discuss.prototype.events, {
         'click .o_mail_mobile_tab': '_onMobileTabClicked',
         'click .o_mailbox_inbox_item': '_onMobileInboxButtonClicked',