Move the innards of G::C::class_name into Gadwall::load()

This is useful to allow programs that call load_app() to load modules
visible to the application without setting PERL5LIB to match what the
application expects.
Add a simple queue table

Note the lack of support for retrying failed jobs.
Add forgotten permissions
Add a prototype queue processor

This consists of a daemon that LISTENs for notifications and scans the
queue table, handing each row off to a Gadwall::Dequeue::* handler to
process. There's not much error handling, other than "don't fall over".
It's simple-minded but robust, to match the queue table.

There's only one handler (::Mail), and it's unimplemented.
Add enqueue_job and enqueue_mail utilities
Make mail and enqueue_mail use a common helper for preparation

This makes it easy to permit either kind of caller the same conveniences
(passing in an array for to and cc, an array of lines for body, etc.).
A very simple implementation of Dequeue::Mail

This assumes that, since the job has been submitted using enqueue_mail
and passed _prepare_mail, it is still fine to be passed on to mail().
That's good enough for now.

In any case, we don't know if we'll continue to use MIME::Lite in the
future. The module is not maintained or recommended (but it sure is a
convenient alternative to the current recommendations!).
Convert the handful of mail() callers to use enqueue_mail()

It occurs to me that there's no reason to make mail refuse to send mail
when MOJO_MODE=testing now: we can just not run dequeued in testing. No
hurry to change that, though.