By default, sort app lists by rating again

Summary: The change to sort app lists by released date by default only works for rolling release distros where app updates are constantly tricking in. For the more common discrete release distros (e.g. Debian, Ubuntu, Fedora, openSUSE Leap), most apps aren't receiving regular updates, and when they do, it's all on the same day. So most of the time, the list is mostly-but-not-quite alphabetical list. We shouldn't regress the user experience for users of these kinds of distros.

Test Plan:
- Deploy the patch
- Launch Discover
- Observe that by default, app lists are sorted by rating again

Reviewers: apol, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11855