[ADD]Event: functionality of Event Mail Scheduler after subscription,bef...

 - Task :https://www.odoo.com/web#id=9355&view_type=form&model=project.task
 - Pad : https://pad.odoo.com/p/r.qB5ryY6Krq83auKa

...ore event and after event

[IMP]add access rule for event.mail and fix typo

[IMP]imporved code and labe

[IMP]updated cron job by 1 hour

[IMP]fix typo, apply group no one

[IMP]change onchange for one2many and set default value for boolean

[IMP]remove unused code

[IMP]add template for event mail schedule and test cases for the event mail scheduler

[IMP]indentation

[IMP]improve the code as per new api

[IMP]imporve the code for send mail
Very good idea to include tests. Thanks !