Unify date and time combos and fix their editing issues

This MR includes new components for the date and time combos used in the incidence editor. This should simplify the incidence editor significantly.

There are also some changes to their functionality to make editing their text and values more consistent with what the user would expect. Previously, their values would change as the user was typing in them, making them confusing and frustrating to use with a keyboard.

This should address issue 13