Overhaul drop chance to a percentage multiplier system

Turning this into a PR because it's easier to follow conversations over multiple commits that way.

I could use input from users interested in how often drops should happen, and statistics experts:

* Is the base chance OK? (about 2.5% for a "value 0" task). It can be adjusted two ways: by changing the divisor so each unit of task value is worth more or less, or by tweaking the subtracted constant to fine-tune the offset.
* Are the values of the factors OK? These are adjusted via the divisors, as you can hopefully see from the examples.
* Is this list of factors OK? Are there any that don't belong, or others we should include?
* PER/10 adds to daily drop cap: yes, no?

I could also use an assist in running a comparative drop chance simulation; I haven't done any tests of the sort yet.