New tripMembers collection

[GrahamGoudeau]

The operation of trip deletion could be very expensive if the number of users grows. The db should be normalized to create a new tripMembers collection, where a document in the collection has the form:

export interface tripMember {
    userId: ObjectIdTs;
    tripId: ObjectIdTs;
};

This would allow a deletion to be done based on the index on tripId.

Also consider making a similar collection for tripOwner.