Itinerary Item association refactor

References: https://github.com/tuxedio/tuxedo-proto/issues/94

This pull requests uses associations which will follow Rails conventions. Previously we were setting foreign keys explicitly, now it will be done implicitly using belongs_to associations.

  i.e. what used to be ```current_trip.itinerary_items.find_by_id(:itinerary_item)``` will just be ```current_trip.itinerary_item.id``` etc.

Also, we can create these associations implicitly now.

  i.e. ``Trip.itinerary_item.create(...)```

<b>As of now:</b>
  1. Tests still need to be written to test the associations are created.
  2. Refactoring of itinerary_items controller, model, etc needs to be done (I think this deserves its own PR)