Don't show deletion reason as a general form field

This one is specifically used next to the delete button, should not be editable
otherwise.
Don't show Preview or Finalize buttons until invoice is saved

Doing a Preview would cause duplicate invoices since clicking back after
the preview would get a new invoice form.

Clicking Finalize without even saving wouldn't cause breakage now, but also
doesn't make much sense, so remove that as well to prevent future bugs.
Add support for sending email reminders of unpaid invoices

In passing, fix some of the messages from the javascript popups, which were actually
using the wrong field and thus not including the email address..
Default to DEBUG=False, unless specified in local_settings.py

It's normally defined in local_settings, but it never hurts to have the
correct default.
Change sender address for invoices to be treasurer@ by default
Prevent deletion/canceling of invoice connected to a system

We don't yet have support for *properly* cancelling invoices generated
from confreg or membership, since those need to make modifications in
the calling systems as well. This needs to be implemented, but for now
just make sure we can't delete it since that would put the system in
a broken state.
Fix date format to be consistent with what's actually sent in the email