Summary:
- use resources for included HTML files
- replace KHTML part with QTextBrowser
- open links to external websites (i.e. Wikipedia) via external web browser
- remove code for downloading and converting Wikipedia page downloads
- change all external links to https
- remove KHTML dependency

Reviewers: yurchor, vkrause

Reviewed By: yurchor, vkrause

Subscribers: vkrause, yurchor, kde-edu

Tags: #kde_edu

Maniphest Tasks: T11573

Differential Revision: https://phabricator.kde.org/D23862