Regenerates priv/gettext/leaf.pot.
Leaf's translatable strings go through a bare t/1 rather than a Gettext
macro, so mix gettext.extract has nothing to hook into — and a host app
can't extract msgids out of a dependency's source anyway. This scans
lib/leaf.ex for t("…") calls and writes the catalog template hosts
merge into their own.
mix leaf.gettext.extractLeaf.GettextPotTest fails when the shipped file is out of date, so a
new string in the template is caught before it ships untranslatable.