Added tests to ensure that when Invitations are valid they have a valid type attribute. Also, changed the tests for cases where any mandatory field is missing, so that they guarantee that the Invitation is not created (because it's not valid). To make that work correctly, some changes in the invitation creation code were necessary. Particularly, for a WebConference we now check if both the start and end dates are present. The form already required them, but now the tests reflect that.

refs #1296