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:
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