Annual loan payment: comprehensive option trigger

- The annual loan payment event has always had an ultra-simplified,
best-case option trigger for "pay back as much as we can."  I've
replaced the trigger with a comprehensive, exact minimum wealth
requirement (not including stewardship bonuses) for all possible loan
combinations using the best available minimum repayment strategy for
each.
- This will properly make the option disabled for the AI and player
when, in practice, it would do nothing different than the first option
(minimum interest payment).  For the player, this is just more correct
and helpful, especially since the player does not get to see what
actually gets paid without manually inspecting their debts afterward.
- This enables more creative repayment strategies or other event options
to be added to the event in the future.  It also provides the foundation
for a "Pay back as much as possible." loan decision to eventually
replace the clutter of now-more-complex tiers of "Pay back X-sized
loan." decisions.