Support multiple units in the `repeat` field

[prncc]

Instead of just single unit fields like repeat: every 60 hours support multi unit fields like repeat: every 2 days and 12 hours.

A little tricky because values can get long and complicated (e.g., 2 months 3 years, 2 weeks 5 days, and 4 hours). To mitigate, units can be reduced to a limited set when writing the field out (years, days, hours), or we could allow non-integer values. The latter is a rabbit hole that adds complexity.

Initially, allow the user to specify repeat periods in this fashion. Later, also save the period in this format.