no way to set scopes for incoming mail

Fixed in stable r13574 <http://bazaar.launchpad.net/~launchpad-pqm/launchpad/stable/revision/13574>.

qa plan: this will need losa help:

 * check <https://qastaging.launchpad.net/+feature-info> shows the feature scope (it does, though the formatting is a bit off wacky)
 * send mail from gmail using a gmail from address (therefore validly dkim signed) to qastaging.l.n
 * losa run incoming.py
 * observe the mail effect (eg bug is created)
 * read the mail log just to confirm it did dkim auth
 * losa add the rule
   mail.dkim_authentication.disabled mail_header:from:martinpool 1 true

(or similar for whoever's testing it)
 * send mail again
 * it should bounce complaining the message is not authenticated
 * the logs should tell you why

... and, at the end of qa, of course remove the rule again.

With the feature rule in place process-mail.py OOPSed:
https://pastebin.canonical.com/50633/ (sorry, internal only, and via
pastebin because it doesn't seem to be available at the usual
place). Here's the traceback though:

    Traceback (most recent call last):
      Module lp.services.mail.incoming, line 380, in handleMail
        signature_timestamp_checker)
      Module lp.services.mail.incoming, line 458, in handle_one_mail
        mail, signature_timestamp_checker)
      Module lp.services.mail.incoming, line 218, in authenticateEmail
        dkim_result = _authenticateDkim(mail)
      Module lp.services.mail.incoming, line 130, in _authenticateDkim
        if getFeatureFlag('mail.dkim_authentication.disabled'):
      Module lp.services.features, line 237, in getFeatureFlag
        return features.getFlag(flag)
      Module lp.services.features.flags, line 229, in getFlag
        return self._known_flags.lookup(flag)
      Module lp.services.features.flags, line 149, in lookup
        v = self._calc(key)
      Module lp.services.features.flags, line 235, in _checkFlag
        if self._known_scopes.lookup(scope):
      Module lp.services.features.flags, line 149, in lookup
        v = self._calc(key)
    TypeError: 'ScopesForMail' object is not callable

I've attached the original message.

Rolled back in r13590.