Add drag and drop to change incidence start times

This MR adds the ability to change an incidence's start time by dragging and dropping in the calendar views.

Month view:

simplescreenrecorder-2021-11-09_19.02.01

Week view:

simplescreenrecorder-2021-11-09_19.02.25

Schedule view:

simplescreenrecorder-2021-11-09_19.02.56