Master calendar chatter message fix jsh

Task: https://www.odoo.com/web#id=9862&view_type=form&model=project.task&action=327
Pad: https://pad.odoo.com/p/openerp-project.task-K961IHH89D

Description: Removing unnecessary notifications in chatter message on Meeting Update. Sending Email to Attendees when Location or Start date of the event is modified.