Fix bug where TENTATIVE events were marked as BUSY instead of BUSY-TENTATIVE (#5467)

Summary:
Fixes #5467

For events where user was an attendee, status was not taken correctly from ATTENDEE element

Reviewers: #free_busy_developers, vanmeeuwen

Reviewed By: #free_busy_developers, vanmeeuwen

Differential Revision: https://git.kolab.org/D194