Create a way to add roles for a user

### Acceptance Criteria
1. A function exists in the UserRolePivotMapper called `addRoleForUser`, which takes in a user id and a string representing a role name and adds that role for that user (if it exists in the `user_roles` table).
1. A RoleService exists that has a function called `addRolesForUser`, which takes in a user entity and an array of strings (role names) and accesses the UserRolePivotMapper's `addRoleForUser` function.
1. Appropriate Exception type is thrown if the role is nonexistent. (This should already work, but verify.)

### Tasks
- Add the `addRoleForUser` function to the UserRolePivotMapper mapper.
- Create the RoleService

Yeah, I guess not. I'll change it to a one-at-a-time thing