Convert custom messaging address fields to Impp fields

Summary:
This does not only allow us to get rid of a lot of error-prone custom
field code in applications, using the designated standard field for this
(standardized in 2007 already!) also makes messaging addresses show up
correctly in Nextcloud for example.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D22447