Overhaul presentation of link buttons in the Reviews section

Summary: Move the link buttons in the {nav 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:
{F5932817}

{F5932816}

{F5932835}

{F5932813}

{F5932812}

Reviewers: #discover_software_store, #vdg, apol

Reviewed By: #discover_software_store, apol

Subscribers: plasma-devel

Tags: #plasma

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