Litters: Add date of pairing and calculate key dates

[Rory-Reid]

Add a "Date of pairing" to a litter".

We can calculate some dates too under an "Estimates" section:

Date	When	Logic
Due date	Pairing date set	Pairing date plus 21-23 Days
Fully weaned	Birth date set	Birth date plus 3.5 weeks
Separate sexes	Birth date set	Birth date plus 4.5 weeks
Earliest rehome	Birth date set	Birth date plus 6 weeks

https://www.nfrs.org/articles_breeding.html

[Rory-Reid]

That's enough of that for now.