The idea is to make it easier for umc devs to implement
their own carryover / early finish bonus mechanics. Maybe
we can also make use of it in Worls Conquest.

And use it in carryover_gold.lua

this way the functions can easily be reused for alternative
carryover implementations.

We now use a modified default carryover calculation,
the main advantage is it now uses the normal carryover
message on endlevel.