Provide links for creating expenses and payments

[thomaseizinger]

Currently, the client knows the endpoints for creating expenses and payments. We should add links to the response of a Nobt that points clients to these endpoints.

{
	"id": "g943hNß57gha",
	...
	"_links": {
		"create-expense": {
			"href": "..."
		},
		"create-payment": {
			"href": "..."
		}
	}
}

Implementation hints can be taken from the "delete" link of expenses. See #8.