Date/time input component

Both the personal calendar editor (`/calendars/personal/edit`) and the event editor (`/events/{EventId:int}/edit`) need a UI component for inputting dates/times. Currently we use HTML ``, but it isn't very user-friendly.

Some options I found:

- https://github.com/flatpickr/flatpickr
- https://flatpickr.js.org/examples/
- https://github.com/Matsuuu/lit-flatpickr
- https://mudblazor.com/components/datepicker
- can't use, as we need it to work without Blazor, but it's nice
- https://web-components.carbondesignsystem.com/?path=/docs/components-date-picker--overview

Start by reviewing the date/time inputs used by /calendars/personal/edit and /events/{EventId:int}/edit, then compare the listed component options and their non-Blazor requirements. Done means both editors use a more user-friendly date/time input component than HTML datetime-local, while remaining usable without Blazor.

Written by the indexing model from the issue text.