party-robot: missing prerequisites

[ErikSchierboom]

The party-robot exercise uses string, the int and float64 types, but its prerequisites don't ensure that the student has learnt these concepts.

The numbers and strings concepts should be added as the prerequisite of the party-robot exercise.

[bobtfish]

This depends on issue #1613 to re-add the strings concept.

With that done the savings-account exercise needs to change to depend on strings rather than string-formatting - which is more correct, and also breaks a dependency cycle we get otherwise. We can then make this change to add these dependencies to the party-robot exercise.