Make the review pop-up a bit more user-friendly

Summary:
Polish the design of the review input pop-up:
- Make the button tell you what you need to do
- Make it impossible to accidentally submit a one-star review

BUG: 390426

Test Plan: {F5712259}

Reviewers: #discover_software_store, apol, abetts

Reviewed By: #discover_software_store, apol

Subscribers: anemeth, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10500