add checkboxes

[Kevin Unger]

• add a checkbox at the top of the add-booking screen "only show free slots"
  -> only shows slots that aren't booked by someone

• add a checkbox at the top of the overview-bookings screen "only show occupied slots"
  -> only shows slots that are booked by someone

after setting one of the two, set setting in settings service
load settings on init

can only be done after #9