The `freeformType` of `settings.publicinbox` in this module prevented
users from setting settings on the `publicinbox` section itself (which
is necessary for making e.g. IMAP work correctly), and only allowed
configuration of nested per-inbox sections.
In general I believe that these overly specific types which are
traditional in NixOS, and this kind of config generation, are a huge
footgun. This commit is the least invasive change that makes the
module work correctly.
aea521150d