sddm-theme: Set needsFullUserModel=false
Summary:
Otherwise SDDM has to load all users even if they're not shown.
See https://github.com/sddm/sddm/pull/1017
Test Plan: Using this for several releases downstream to support >1500 users.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: ngraham, davidedmundson, filipf, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D24333