Replace weekday-number methods with a single days_since method

Essentially:

```
number_from_monday() => days_since(Monday) + 1
number_from_sunday() => days_since(Sunday) + 1
num_days_from_monday() => days_since(Monday)
num_days_from_sunday() => days_since(Sunday)
```

If we want to replace `number_from` with something I'd probably go with `days_since_plus_one` or something more explicit.