Monthly frequencies should have their own representation reflecting their special behavior

[hiqua]

Given: 33468bf and #947

It looks like whenever the denominator is 30 or 31, we consider this to be a monthly habit. In this case, we compute frequencies differently depending on the month we're in. Right now, the FrequencyPickerDialog is just an interface helper to easily select numerators and denominators, e.g. "every day" is 1/1, and we encode every frequency this way.

I think we should reflect in the interface that monthly habits behave differently from all others, so selecting n times per month. I also think we should reflect in the actual data that monthly habits are special, instead of using 30/31 as special values, and we should still allow users to specify n times every 30 days (!= every month) if they wish to do so.