Polish the reviews UI and presentation

Summary: Implement mockups provided by VDG to improve visual appeal of reviews UI. Layout is a bit cramped due to https://bugs.kde.org/show_bug.cgi?id=415677.

Test Plan:
{F7849164}

{F7849165}

{F7849166}

Reviewers: #vdg, apol, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25889