Add support for goals

[morremeyer]

Is your feature request related to a problem? Please describe.
The backend has started supporting goals, so the frontend should support them, too.

Goals describe that a certain amount of money should be allocated to an envelope at a certain month.

This can

Describe the solution you'd like

Have an integration of goals in the frontend that allows them to be easily viewed and updated

Additional context

Related to:

• Add recurrency for goals backend#904, which adds recurring goals
• Add allocation proposal endpoint backend#903, which adds allocation recommendations based on goals