[KCMs/Feedback] Adjust UI to be more user-friendly

Summary:
Adjust to the string changes in D26551 and improve the presentation
overall to make it more likely that people will feel comfortable
using it and submitting information.

Depends on D26551

Test Plan: How it looks now:

Reviewers: VDG, Plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26552