Add 2 week grace period for new users

[ConcernedHobbit]

If a new user's application has not been yet accepted, we have a 2 week grace period in members calendar event registration to allow freshers to join.

Should this be system-wide, i.e. user-service returns something other than non-member, so that freshers can also use e.g. tärpistö? Might also be useful to be able to restrict the services available, e.g. arkisto should probably be viewable only to members accepted by the board.