CC-01: Gift Models

[matherg]

Create type definitions in the type.go file based off our UML diagram for Gifts, GiftRequest, GiftResponse and GiftCollection.
Create input types for each model.
Create relations between GiftRequest - GiftResponse (1:1), GiftCollection-GiftResponse(1:0,1), GiftCollection - Gift (Many:Many)
Make sure to place constraints where needed (ex. age should be between 0 and 120)
Test correct model creation and relations in the db_test.go file.

@aidanobrien411 @DamianUduevbo
Gorm
EngNotes
UML