[FIX] mail: restrict mention suggestions to channel members

Problem is that when someone is mentionned in a channel he doesn't belong to
(public, private or DM whatever), the message only appears in its Inbox and
he can't know where it comes from nor reply. This is thus very confusing.

With this rev., in Discuss, one can only mention members of the channels.
In the chatter, anyone can be mentionned like before, as when the message
arrives in the Inbox, there is a link to the associated document.

addons/mail/static/src/js/chat_manager.js

@@ -791,7 +791,7 @@ var chat_manager = {
                         }));
                     });
 
-                    return [members].concat(suggestions);
+                    return [members];
                 });
         }
         return channel.members_deferred;

addons/mail/static/src/js/client_action.js

@@ -171,8 +171,8 @@ var ChatAction = Widget.extend(ControlPanelMixin, {
         this.searchview = new SearchView(this, dataset, view_id, {}, options);
         this.searchview.on('search_data', this, this.on_search);
 
-        this.basic_composer = new composer.BasicComposer(this);
-        this.extended_composer = new composer.ExtendedComposer(this);
+        this.basic_composer = new composer.BasicComposer(this, {mention_partners_restricted: true});
+        this.extended_composer = new composer.ExtendedComposer(this, {mention_partners_restricted: true});
         this.thread = new ChatThread(this, {
             display_help: true,
             shorten_messages: false,

addons/mail/static/src/js/composer.js

@@ -331,6 +331,7 @@ var BasicComposer = Widget.extend({
             input_max_height: 150,
             input_min_height: 28,
             mention_fetch_limit: 8,
+            mention_partners_restricted: false, // set to true to only suggest prefetched partners
             send_text: _('Send'),
         });
         this.context = this.options.context;
@@ -620,7 +621,7 @@ var BasicComposer = Widget.extend({
                     }
                 }
             });
-            if (!suggestions.length) {
+            if (!suggestions.length && !self.options.mention_partners_restricted) {
                 // no result found among prefetched partners, fetch other suggestions
                 var kwargs = {
                     limit: limit,