akregator: install kontactplugin into kontact5, with JSON metadata.

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

Test Plan: make uninstall, apply patch, make install, kontact loads akregator plugin

Reviewers: winterz, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28610