DateTimeField in process_form_data is passed through without processing; convert to ISO 8601

While the `FormData` formats of `DateField` (`2024-02-24`) and `TimeField` (`19:55`) suffice and are close enough to ISO 8601, the format of `DateTimeField` is not (`2024-02-24T19:55` without seconds or time zone). In `form_data` it should be what the user's browser sent us, but in `values` it should be converted first to the full ISO 8601 UTC format of `2024-02-24T17:55:00Z` and finally, as of #436, to the scope time zone such as `2024-02-24T19:55:00+02:00`.

The `` does not convey the time zone but implies it is the user's time zone. In places where we send `formData: Object.entries(formData)`, we should probably include our best guess of the user time zone either added into the `formData` or next to it. (OR: Should we understand this to be always in the scope time zone? Case US organizer of a FI event setting the program schedule publishing time)

There are occasions of `TODO(#436)` in the frontend source code signalling places where we could be using `DateTimeField` if this was taken into account, but currently are not.