Install the kontact plugin into kontact5, with JSON metadata.

Summary: This will allow kontact to use KPluginLoader one day.

Test Plan:
rm $prefix/lib64/plugins/kontact_zanshinplugin.so ; make install
kontact still finds the zanshin plugin

The same steps (other than old-kontact-interface compatibility) were already
pushed to all other kontact plugins.

Reviewers: ervin

Reviewed By: ervin

Differential Revision: https://phabricator.kde.org/D28847