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. Fixing bug of which was allowing user to write start date after end date.