Allow adhocracy.registration_support_email as from.

When using different email adresses for registration and general
support, for some mass messages it is more appropriate to use the
registration support address. Specifically, invitations.
Fixes hhucn/adhocracy.hhu_theme#353.
PEP-8
fix typo
Merge pull request #438 from wawe/feature-use-registration-email-in-massmessage

Allow adhocracy.registration_support_email as from.