savings-account: tweak design

[ErikSchierboom]

The savings-account exercise is has a very basic "theme". This has a couple of issues:

1. The tasks don't naturally link to the story/theme.
2. It is quite dull. There are probably not many programmers that would find the tasks interesting.

We should come up with a theme and modify the tasks to make more sense.

The const keyword itself is explained in the basics concept, so this exercise can delve deeper into the more advanced usages of constants (like numeric constants, iota, etc.)

This is basically the same issue as described in #1500 (comment).

[junedev]

We will make these improvements part of #2226 so closing this issue to avoid duplication.