Prevent to send web notifications to other users

Only the admin user (sudo) is allowed to send notifications to other
users. The normal users can only send notifications to themselves.

This is to prevent attackers to craft malicious notifications and send
them to other users using RPC.

Correction based on the idea of @hbrunn

web_notify/models/res_users.py

@@ -1,7 +1,7 @@
 # Copyright 2016 ACSONE SA/NV
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
 
-from odoo import api, fields, models, _
+from odoo import api, exceptions, fields, models, _, SUPERUSER_ID
 
 
 class ResUsers(models.Model):
@@ -35,6 +35,11 @@ class ResUsers(models.Model):
 
     @api.multi
     def _notify_channel(self, channel_name_field, message, title, sticky):
+        if (self.env.uid != SUPERUSER_ID
+                and any(user.id != self.env.uid for user in self)):
+            raise exceptions.UserError(
+                _('Sending a notification to another user is forbidden.')
+            )
         bus_message = {
             'message': message,
             'title': title,