no intermediary parsing step. Also, moved the date -> duration (3mo6d3h
sorta thing) to the web, there's now a ValidTill field with a unix
timestamp. Used the new Temporal api with a polyfill. Bumped api version
(although it still isn't semver).
customcontent.go constains a structure with all the custom content,
methods for getting display names, subjects, etc., and a list of
variables, conditionals, and placeholder values. Tests for constructX
methods included in email_test.go, and all jfa-go tests can be run with
make INTERNAL=off test.
