[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

4 files changed, 12 insertions, 8 deletions

diff --git a/addons/mail/static/src/js/models/messages/message.js b/addons/mail/static/src/js/models/messages/message.js
index 1176c0f8c48..4342b1e244a 100644
--- a/addons/mail/static/src/js/models/messages/message.js
+++ b/addons/mail/static/src/js/models/messages/message.js
@@ -458,6 +458,9 @@ var Message =  AbstractMessage.extend(Mixins.EventDispatcherMixin, ServicesMixin
      */
     setModerationStatus: function (newModerationStatus, options) {
         var self = this;
+        if (newModerationStatus === this._moderationStatus) {
+            return;
+        }
         this._moderationStatus = newModerationStatus;
         if (newModerationStatus === 'accepted' && options) {
             _.each(options.additionalThreadIDs, function (threadID) {
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;
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];
         });
     },
     /**
diff --git a/addons/mail/static/src/js/models/threads/thread.js b/addons/mail/static/src/js/models/threads/thread.js
index c9dda28dd93..8c2beb8bbfa 100644
--- a/addons/mail/static/src/js/models/threads/thread.js
+++ b/addons/mail/static/src/js/models/threads/thread.js
@@ -15,7 +15,8 @@ var ServicesMixin = require('web.ServicesMixin');
  * In particular, channels and mailboxes are two different kinds of threads.
  */
 var Thread = AbstractThread.extend(ServicesMixin, {
-
+    // max number of fetched messages from the server
+    _FETCH_LIMIT: 30,
     /**
      * @override
      * @param {Object} params
@@ -33,8 +34,6 @@ var Thread = AbstractThread.extend(ServicesMixin, {
         // means that there is no message in this channel.
         this._previewed = false;
         this._type = params.data.type || params.data.channel_type;
-        // max number of fetched messages from the server
-        this._FETCH_LIMIT = 30;
     },
 
     //--------------------------------------------------------------------------
@@ -111,7 +110,7 @@ var Thread = AbstractThread.extend(ServicesMixin, {
      * By default, a thread has not listener.
      *
      * @abstract
-     * @returns {$.Promise<Object[]>}
+     * @returns {$.Promise<Array<Object[]>>}
      */
     getMentionPartnerSuggestions: function () {
         return $.when([]);