Out-of-office support for assignees

Assignees should be able to mark themselves as unavailable for a certain period of time, so that new items do not get automatically assigned to them, and so that others can see if urgent items are assigned to people that are unavailable and can reassign if required.

For reference, see the respective functionality in t4db/hooligan, where this feature was an extremely useful.

Ideally this would be integrated with some centralized calendaring (Exchange) so that it is enough for users to log out-of-office time in one calendar (sync-back from JIRA to Exchange would not be necessary).