Overhaul presentation of link buttons in the Reviews section

Summary: Move the link buttons in the Reviews section around to improve their presentation. In the process, allow people to write reviews of installed apps without having to first open the review pop-up.

Test Plan:

Reviewers: Discover Software Store, VDG, apol

Reviewed By: Discover Software Store, apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D13678