Implement tiered interest margin lists

[bosue]

For various reasons, interest margins are usually tiered by loan amount. Large loans may be just a bit cheaper than small loans:

I initially implemented a proof of concept of this feature in #54, however per discussion in #19, we want to strip this overly large and slightly complex PR to a minimum viable product and introduce additional features in separate tickets.

Implementation should be as follows:

• Allow the back office to define a bandwidth of interest margins to be available for Loan Products of this Loan Type.
• Allow the product / sales manager, when defining a Loan Product, to either pick a particular margin for a rather standardized product or to stick with the bandwidth for full flexibility on the Loan Application side or to further reduce the bandwidth appropriately. Expanding it should obviously not be supported on the Loan Product side.

All of this per tier, so administrative and refinancing costs may be appropriately reflected in the resulting end price (interest rate).

Obviously, implementation is postponed on #54.