easy screens maintenance strategy

From https://bugs.launchpad.net/ubuntu/+source/ubuntu-docs/+bug/1573025/comments/5

> Screenshots add to the maintenance burden, and the desktop guide only contains a few screenshots. We simply don't have the resources to include screenshots everywhere.

This is very unfortunate, because people read visual information several magnitudes times faster. I sense three problem with the maintenance of screens:

1. new versions of software are released
2. referenced software getting replaced with alternatives
3. no place to upload screens

Replacing old screens is not needed - just a link needs to be updated.

The solution to this problem is to augment docs with some https://screens.ubuntu.com/ CDN where people with Ubuntu logins can upload and review screens for software distributed with Ubuntu. Making this CDN available will also allow people to implement search for bug targets using screens shots.