fix error when %recipients is empty

https://bugzilla.mozilla.org/show_bug.cgi?id=904467
add order to extension paths.

it's better to be explicit than to rely on glob() magic.
some extensions have side effects: order matters.
some extensions need to be first / last.

this adds a @extension_paths setting in localconfig.
(similar to django's settings.INSTALLED_APPS)

https://bugzilla.mozilla.org/show_bug.cgi?id=908087
add extension as a submodule
this will need commit a997d023e68dd9b01f8e6921527a4d1347244b2e as a pre-req.

https://github.com/dnozay/Bugzilla-Extension-MultipleEditNoEmail
https://bugzilla.mozilla.org/show_bug.cgi?id=23924