[FIX] mail: fix several traceback from a recent refactoring

Revision on https://github.com/odoo/odoo/commit/02ec09cb1c3e2d7bc7968f40c18f2208d7f3f498

This commit fixes some leftovers from an intermediary state of the refactoring above.

The following tracebacks should be fixed:
    - unsubscribe from a private channel
    - replay
    - inviting a user

addons/mail/static/src/js/discuss.js

@@ -99,7 +99,7 @@ var PartnerInviteDialog = Dialog.extend({
                     self.do_notify(_t('New people'), notification);
                     // Clear the membersDeferred to fetch again the partner
                     // when getMentionPartnerSuggestions from the chatManager is triggered
-                    var channel = this.call('chat_manager', 'getChannel', self.channelID);
+                    var channel = self.call('chat_manager', 'getChannel', self.channelID);
                     delete channel.membersDeferred;
                 });
         }
@@ -508,7 +508,7 @@ var Discuss = Widget.extend(ControlPanelMixin, {
      */
     _selectMessage: function (messageID) {
         this.$el.addClass('o_mail_selection_mode');
-        var message = this._getMessage(messageID);
+        var message = this.call('chat_manager', 'getMessage', messageID);;
         this.selected_message = message;
         var subject = "Re: " + message.record_name;
         this.extendedComposer.set_subject(subject);
@@ -903,10 +903,10 @@ var Discuss = Widget.extend(ControlPanelMixin, {
         if (def) {
             def.then(function (value) {
                 if (value !== 'granted') {
-                    self._sendNotification(self, _t('Permission denied'),
+                    self.call('bus_service', 'sendNotification', self, _t('Permission denied'),
                         _t('Odoo will not have the permission to send native notifications on this device.'));
                 } else {
-                    self._sendNotification(self, _t('Permission granted'),
+                    self.call('bus_service', 'sendNotification', self, _t('Permission granted'),
                         _t('Odoo has now the permission to send you native notifications on this device.'));
                 }
             });
@@ -963,7 +963,7 @@ var Discuss = Widget.extend(ControlPanelMixin, {
      * @private
      */
     _onUnsubscribeButtonClicked: function () {
-        this._unsubscribe(this.channel);
+        this.call('chat_manager', 'unsubscribe', this.channel);
     },
 });