Staff: add the ability for staff members to cancel an absence before it occurs

[SKuipers]

Currently, if staff members submit an absence and something comes up where it's no longer necessary, they would need to contact an admin to delete it for them. However, when this happens first thing in the morning when arranging staff coverage, this can be problematic. This feature adds the ability for staff members to cancel an absence, including any attached coverage requests, as long as it is on or before the date of the absence. Doing so sends a message to all parties involved about the cancelled absence.

The main changes are:

• Adds a new Cancelled status to gibbonStaffAbsence, and implements in existing queries & tables
• Adds a "Cancel" button to the View Absences page for a staff member
• Adds new messages for AbsenceCancelled and AbsenceWithCoverageCancelled
• Also updates the existing coverage cancellation option so it can be used to cancel requested coverage without cancelling the absence.

How Has This Been Tested?
Locally, and soon to be tested in production.