Fix for Bug #1296

* Fixed the error with the .ics file format. Instead of sending the iCal of the event directly, we now first add it to a calendar, then send it.
* Also, some errors were noticed: the creation of the invitations was being done with the more Generic class instead of the specific ones. Fixed that.
* Added tests to ensure that an invitation always has its type set.
* Changed the invitation email of the events to match the one of web conferences. For that, a partial view and the necessary i18n resources were created.
* Made the lack of dates (in invitations for Web Conferences) and of title (in invitations for Events) produce an error. Added tests for that.