New port: www/riot-web: Glossy Matrix collaboration client for the web

Riot (formerly known as Vector) is a Matrix web client built using the
Matrix React SDK.

Riot is officially supported on the web in modern versions of Chrome,
Firefox, and Safari. Other browsers may work, however official support is
not provided. For accessing Riot on an Android or iOS device, check out
riot-android and riot-ios - riot-web does not support mobile devices.

WWW: https://github.com/vector-im/riot-web

PR: 240972
Submitted by: Alexander Sieg <alex@xanderio.de>