Add photographic user avatars

Summary:
This patch adds 20 photographic avatars in order to finish work on T9910.

Sylistically they are simple and with only a single topic in focus.

Sizes are 512x512, the formats .png. They've been tested with our circular avatar template and cropped to fit it well.

All are crops of strictly CC0 licensed images from Pexels. License information has also been supplied.

Closes T9910

Test Plan:

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: jriddell, cblack, ognarb, plasma-devel

Tags: Plasma

Maniphest Tasks: T9910

Differential Revision: https://phabricator.kde.org/D29394