Option to specify default/home channel which is shown instead of home page `config.json` …

From https://github.com/vector-im/element-web/blob/develop/docs/config.md …

> 17. embeddedPages: Configures the pages displayed in portions of Element that embed static files, such as:
[…]
homeUrl: Content shown on the inside of the app when a specific room is not selected. By default, no home page is configured. If one is set, a button to access it will be shown in the top left menu.

It would be helpful if we could define a default/home channel, too. Instead of showing the home page after logging in or after removing the `room/…` part from the URL, the specified channel would be shown. That channel might be read only and filled with content/information. Admins/Mods without access to the server could change the content here.