Allow rooms to be viewable without signing in

[MadLittleMods]

Is your feature request related to a problem? Please describe.

A little hidden feature in Element is that you can visit a room directly without signing in and view messages: https://app.element.io/#/room/#matrix:matrix.org - This is a very nice frictionless flow to view content in a room when you're coming from a matrix.to badge on a readme or just sharing the link directly to what you're looking at.

Here is how the not logged in view looks like in Element:

Describe the solution you'd like

Allow rooms to be viewable without signing in. This is different from guest login (ILAG, improved landing as guest) and I'm not suggesting that here. I just want to view a room without signing in.

The URL's in Hydrogen are a bit different since they include your session ID in the URL which isn't very shareable. But we could easily redirect if someone isn't signed in to any accounts. And if someone is signed in, ask them to choose the session to open the room in instead of showing "invalid session ID"

https://hydrogen.element.io/#/session/225393722894987/room/!OGEhHVWSdvArJzumhm:matrix.org

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context

Hydrogen could be a good mobile web alternative to Element Web which doesn't have mobile web support (related element-hq/element-web#19870)

[bwindels]

If it helps, You can already use an alias for the last used session id, like so:

https://hydrogen.element.io/#/last-session/room/!OGEhHVWSdvArJzumhm:matrix.org

[MyWay]

Also, when embedded on a website, most users will leave without registering. A 'preview', or a guest-mode would certainly help.