Enforce Date typing

[jorgengaldal]

When dealing with dates, both Date objects and strings are used, and there is a lot of casting between the two types.
We should consistently use Date objects throughout the codebase (including in the database).

[jorgengaldal]

Date-objects are used here:

• In MongoDB:
  • Applications.date

Strings are used here:

• In MongoDB:
  • Committees.availabletimes.start/.end
  • Interviews.interviews
  • Period.applicationPeriod
  • Period.interviewPeriod
  • Rooms.startDate
  • Rooms.endDate

[julian-ao]

By fixing this, the timezone bug may magically be fixed