Support room IDs and event permalinks in the join command
Mostly useful for power users who want to jump to places more easily.
Examples:
* `/join !somewhere:example.org`
* `/join !somewhere:example.org altserver.com` - to join through `altserver.com`
* `/goto https://matrix.to/#/!somewhere:example.org/$something:example.org?via=altserver.com`
Fixes https://github.com/vector-im/riot-web/issues/3248
Fixes https://github.com/vector-im/riot-web/issues/7543