Handle all the segments of a v3 event ID
They may contain slashes, so it is not suitable to just pull the first segment after the room ID. Instead, we just recompile the event ID from known source, assuming everything afterwards is an event ID.
Fixes https://github.com/vector-im/riot-web/issues/8315
This will need adapting to support https://github.com/vector-im/riot-web/issues/9149
