[Frontend] - Fix assignment dates

[LambrechtMaarten]

• When creating an assignment, the HTTP.post would automatically convert this to UTC. Because of this when a startDate/deadline was created at 00:00 in a time zone like Brussels with +2 hours. This date would be one day earlier at 22:00.
• I fixed this by converting to date to noon (12:00) on that day, this way the UTC conversion does not change the day of the date.
• This is a fix that can be done in a cleaner way, by working with Date objects in a more consistent way. But for now this should suffise because we don't store hours in our DB for startdates/deadlines

[bramcomyn]

Good fix!