Only let the user write the first review if they've actually installed the app

Summary: Previously, you could write the first review for an app even if you didn't have it installed. No longer, with this patch!

Test Plan: {F5712354}

Reviewers: #discover_software_store, apol

Reviewed By: #discover_software_store, apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10525