Add "user-others" icon

Summary:
BUG: 407782

Needed to represent permissions for other users.
{F7118835}

Test Plan: Search for `user-others` in Cuttlefish.

Reviewers: #vdg, ndavis, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, kde-frameworks-devel, #vdg

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D22822