Mute avatars and read receipts on event tiles
This reduces overall noise from the screen reader. It was reading the alt attribute from the sender avatar, which was just a mxid. The read receipts were just nonsensical noise.
Fixes https://github.com/vector-im/riot-web/issues/2716
Fixes https://github.com/vector-im/riot-web/issues/5697
See https://github.com/vector-im/riot-web/issues/9747
