Add footer_sender_exceptions option (T850567)

Summary: Example: footer_sender_exceptions = NONE, *.kolab.ch, kolab.ch

Reviewers: PyKolab Developers, mollekopf

Reviewed By: PyKolab Developers, mollekopf

Subscribers: mollekopf, PyKolab Developers

Differential Revision: https://git.kolab.org/D4463