Editing shifts starting at midnight uses wrong date

[lukasrad02]

Describe the bug

When a shift that starts ad midnight on day X is edited, the date form field is prefilled with date X-1 and saved like this. Hence, editing some other property of such a shift also alters the date.

To Reproduce

Steps to reproduce the behavior:

1. Create an event with a single shift starting on day X at midnight (12 am) and ending at, for example, 1 am.
2. Save the event and shift. Check that the event date and shift date displayed on the event's page are equal to date X.
3. Edit the shift. Notice that the date field shows X-1.
4. Save the shift without making any changes.
5. The shift (and thus the event) have now moved to date X-1, as can be seen on the event's page

Expected behavior

When editing a shift, the form should be prefilled with the correct current values of the shift so that the only changes that will be applied to the shift are those the user explicitly entered into the form.

Environment

Tested with Ephios v0.17.2 as well as the latest commit on main (9be8059). Bug occurs independent from the browser used (tested with Firefox and Chromium).

Additional context

Might be related to #515