Impacted versions:
11.0
### Steps to reproduce:
- ### Create a calendar event from a document (sale order, invoice, applicant ..)
![image](https://user-images.githubusercontent.com/10233975/42788124-54906394-8923-11e8-875d-dad7d158d7e2.png)
- ### Edit it and add new guest

![image](https://user-images.githubusercontent.com/10233975/42788143-6a8e5da4-8923-11e8-9447-00022b3d47fd.png)


- ### Accept / Open the invitation from guest mail
![image](https://user-images.githubusercontent.com/10233975/42788165-829ef4e4-8923-11e8-8b01-fecf0aca7986.png)

### Current behavior:

![image](https://user-images.githubusercontent.com/10233975/42788178-8fca413c-8923-11e8-97f9-5d2d8043b369.png)

![image](https://user-images.githubusercontent.com/10233975/42788181-96ed1570-8923-11e8-8804-f94f3a093f34.png)

Expected behavior:
Should render the invitation page.

http://348861-11-0-979b2e.runbot16.odoo.com
Replicated at http://runbot16.odoo.com/runbot/static/build/348861-11-0-979b2e/logs/job_30_run.txt


## The thing is that the event has to have a description in order to render correctly the invitation page.
![image](https://user-images.githubusercontent.com/10233975/42788218-eab9b082-8923-11e8-948e-6301dfb9d95b.png)
![image](https://user-images.githubusercontent.com/10233975/42788220-f009f920-8923-11e8-8244-50c2a4324202.png)

Video/Screenshot link (optional):

OPW 1867283
_inherit = "mail.activity"
    calendar_event_id = fields.Many2one('calendar.event', string="Calendar Meeting", ondelete='cascade')
    @api.multi
    def action_create_calendar_event(self):
        self.ensure_one()
        action = self.env.ref('calendar.action_calendar_event').read()[0]
        action['context'] = {
            'default_activity_type_id': self.activity_type_id.id,
            'default_res_id': self.env.context.get('default_res_id'),
            'default_res_model': self.env.context.get('default_res_model'),
            'default_name': self.summary,
            'default_description': self.note and tools.html2plaintext(self.note) or '',
            'default_activity_ids': [(6, 0, self.ids)],
        }
        return action
    def action_feedback(self, feedback=False):
        events = self.mapped('calendar_event_id')
        res = super(MailActivity, self).action_feedback(feedback)
        if feedback:
            for event in events:
                description = event.description
                description = '%s\n%s%s' % (description or '', _("Feedback: "), feedback)
                event.write({'description': description})
        return res
    def unlink_w_meeting(self):
        events = self.mapped('calendar_event_id')
        res = self.unlink()
        events.unlink()
        return res