Port phone numbers Grantlee model to use KContact gadgets directly

Summary:
This is also moving link assembly from the model to the template, which
is where it belongs IMHO. Template changes follow shortly.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20545