Ship plasma-logo in repo

Summary:
We don't want the plasma-logo on the homepage to be changed to something else by
the icon theme. It's not used like an icon but as a logo and should always
display our brand.
BUG: 415173

Test Plan: Open systemsettings with different icon and color themes

Reviewers: Plasma, GB_2, ngraham

Reviewed By: GB_2, ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26078