Add confirmation section in the invitation email

[acotiuga]

When creating a new event in calendar and send it by email to other guests, they receive along with the calendar event preview, a section to confirm the participation to the event. This will not be updated in the meeting, but it would help on the calendar side.
In other words:
Current view, without confirmation section:

Possible improvement, with confirmation section:

This needs to be investigated, to see how extra info can be added to the event.

[arcilli]

Seems like this functionality is implemented for Google Calendar users. More precisely, the second screenshot is generated by Google Calendar when an user receives an invitation for an event stored on owner's (Google) Calendar.
When the invited user accepts the invitation, an entry is made in his (Google) calendar with the details of the event.

In our case, the application is not connected with (Google) calendar, because the event is stored on XWiki pages. Shifting to Google would cause a few other issues (e.g. privacy, user's data, generate a calendar for each wiki user).

[acotiuga]

In this case a better approach is to have a confirmation mechanism inside XWiki, inside the meeting.
There could be 2 list of users stored in a meeting to keep the confirming and denying users.