[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/channel.js b/addons/mail/static/src/js/models/threads/channel.js
index 695275e799e..808e8b7fbca 100644
--- a/addons/mail/static/src/js/models/threads/channel.js
+++ b/addons/mail/static/src/js/models/threads/channel.js
@@ -179,7 +179,8 @@ var Channel = SearchableThread.extend(ThreadTypingMixin, {
     /**
      * Get listeners of a channel
      *
-     * @returns {$.Promise<Object[]>} resolved with list of channel listeners
+     * @returns {$.Promise<Array<Object[]>>} resolved with list of list of
+     *   channel listeners.
      */
     getMentionPartnerSuggestions: function () {
         var self = this;
@@ -193,7 +194,7 @@ var Channel = SearchableThread.extend(ThreadTypingMixin, {
             })
             .then(function (members) {
                 self._members = members;
-                return members;
+                return [members];
             });
         }
         return this._membersDef;