Allow multiple Chatrix blocks on the same page or on different browser tabs

[psrpinto]

This PR brings in the changes from hydrogen related to "same session in multiple tabs" feature. The feature flag needs to be manually enabled through Settings, for now.

Enable feature flag

1. Open a session
2. Click the Settings gear icon (or three-dot menu and then Settings, when in single-room mode)
3. Scroll down to Experimental Features
4. Enable the Open the same session in multiple tabs feature flag

[ashfame]

Looks good 👍

I am unclear on how to enable the feature flag though. I imagine, we would enable the feature flag for chatrix build and we would have it functional without making any changes in the settings. What am I missing?

[psrpinto]

I am unclear on how to enable the feature flag though

The idea was that, for now, the user needs to go into Hydrogen's settings and enable the feature flag, since some things are broken, for example logout, and encrypted rooms, etc. Once we've address (some of?) these issues we can issue a new release with the feature flag enabled by default. Does that make sense?