Need to update Prisma model

[yourvivian]

Potentially updating schema.prisma

• A trip has many activities, so we should reflect that relationship here. You may find this helpful
• When we delete a trip or modify it, we need to ensure that all the relationships are properly updated.
• For a 1-to-many relationship, deleting a trip means that all associated activities should be deleted. The native way to do it using Prisma is with referential action
• This means that if you allow PUT /trips/:id to modify activities and participants, you should check whether the relationship is consistent after the API call.

Originally posted by @nickbar01234 in #7 (review)