This sets a default-but-query-configurable Faker seed in development,
via faker-seed. It also changes uses of Math.random to use Faker’s
randomness so auto-generated data remains stable in development.
I noticed while working on #6166 that some of the factory properties
that used Faker’s randomisation features are using their output
rather than a function that would call the randomiser. This means that
the randomisation happens once and the value is used for every model
generated by the factory. This wraps the randomiser calls in functions
so different models can have different values.
