The per-AI "start_date" is a lot of custom code, and was rarely
used in the way it was meant.
While at it, also ported this part over to the new timer system.
And set the minimum maximum loan to the value of loan interval, so there is
always an amount of money to lend. Compared to being allowed to set max loan
to 0 and never be allowed to lend any money.
This reduced the load on compilers, as currently for example MacOS
doesn't like the huge settings-tables.
Additionally, nobody can find settings, as the list is massive and
unordered. By splitting it, it becomes a little bit more sensible.