[MERGE][FIX] mail: several fixes in discuss, chat window, chatter

This merge fixes a crash in discuss, in addition to re-introduce some _quality of life_
features that have been mistakenly removed from the chatter and from chat windows:

### 1. No crash when searching messages

This was caused by the addition of moderated channels. In short, fetching twice
the same message was performing a re-render of the thread due to a mis-consideration
of a moderation status change. When the fetch reaches the fetch limit, the re-render is
considered as a "load more" fetch, which only works when there is at least one message
registered for the current search.

This is fixed by re-render when there is a change of moderation status only.

Task-ID 1910180

### 2. Easily mention followers and employees in chatter

When mentioning a user from the chatter in a document, it was always
fetching all partners in a arbitrary order. As a result, it was hard to mention
followers and employees.

This is fixed by re-introducing the previous behaviour on mentions, which
groups them by followers, then employees, then all partners.

Task-ID 1910111

### 3. Show message document link in chat window

When a message is linked to a document, it usually shows a
link next to the message. This still works in the Discuss app,
but not in chat windows.

This erroneous regression was caused by a refactoring.
This is fixed by re-introduced document links on messages in chat
windows.

Task-ID 1910119

closes odoo/odoo#28744

1 files changed, 3 insertions, 2 deletions

diff --git a/addons/mail/static/src/js/models/threads/livechat.js b/addons/mail/static/src/js/models/threads/livechat.js
index b7235e7bc0e..f4cb58ebad3 100644
--- a/addons/mail/static/src/js/models/threads/livechat.js
+++ b/addons/mail/static/src/js/models/threads/livechat.js
@@ -37,7 +37,8 @@ var Livechat = TwoUserChannel.extend({
      * display of a user that is typing.
      *
      * @override
-     * @returns {$.Promise<Object[]>} resolved with list of livechat members
+     * @returns {$.Promise<Array<Object[]>>} resolved with list of list of
+     *   livechat members.
      */
     getMentionPartnerSuggestions: function () {
         var self = this;
@@ -49,7 +50,7 @@ var Livechat = TwoUserChannel.extend({
                     name: self._WEBSITE_USER_NAME,
                 });
             }
-            return self._members;
+            return [self._members];
         });
     },
     /**