Persist: add cascade rules to foreign keys

It turns out persist does support these, they're just not mentioned in
the yesod book.

(for future reference, the syntax is defined in Database.Persist.Quasi,
which does include some documentation [1])

[1] https://hackage.haskell.org/package/persistent-2.14.6.1/docs/Database-Persist-Quasi.html

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