Allow option to un-decline a meeting when a clashing meeting is cancelled
1) I accept a meeting
2) A second clashing meeting comes in.
3) I decline the first meeting as the second one is more important
4) The second meeting is rescheduled or cancelled
5) Now, there should be a way to 'get back' whatever meeting I declined in that slot. Both an indication in the calendar UI that something used to be there and was declined, and a way to re-accept that deleted thing.