Don't use a scrollview to display package changelogs

Summary:
BUG: 387036

Don't put the package changelogs in a scrollview, because it introduces some usability issues (see the bug for details).

Also change the "Open" button to day "More Information..." since it wasn't clear what you were opening.

Test Plan:
Tested in KDE Neon. Package changelogs are now fully visible without being in a tiny scrollview:

{F5495553}

Reviewers: #discover_software_store, apol

Reviewed By: #discover_software_store, apol

Subscribers: abetts, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8869