Default SDDM and KScreenLocker backgrounds

[pointedstick]

Howdy Plasma folks! VDG has managed to achieve a consensus regarding a desired change in the default SDDM and KScreenLocker backgrounds. The current default is a flat blue color; we would like to change them both to use the current default Plasma background.

We believe this addresses all concerns:
- Aesthetics: all involved agreed that the Plasma wallpapers are nicer than the flat blue color
- Privacy: no issue, since no custom user data is leaked
- Implementability: image is not dynamic and has no effects applied to it; no problem trying to do anything complicated that would run into a million corner cases
- Branding: after a Plasma version upgrade, users who have not changed any default settings would automatically see the new wallpaper on their lock and login screens

The following Phabricator task tracks this work: https://phabricator.kde.org/T7914

Thanks folks! Let me know if there are any questions or concerns.

Nate