Allow Loan Partner shareables to have a minimum (partner) amount

[bosue]

As implemented in #35, Loan Partner currently supports shareables in terms of ratio or percentage shared.

Often, loan partners will instead expect a fixed amount or lump sum of a particular shareable revenue.

It would be nice if the Loan Partner Shareable doctype could be amended to allow fixed amounts as well.

[bosue]

Even better idea, IMO:

Let us implement this as a minimum amount, so it combines with partner percentage (or ratios).

With a partner percentage of 0, your minimum amount would be equivalent to a fixed amount.

Additionally, you would have the option to implement, say, a partner percentage of 20%, but at least USD 25.

UI can cope with this and still make it super easy to use.