Add option for session sharing

[MoritzWeber0]

On the session card in "Active sessions", there should be a button "Share" if the connection method supports sharing.
This is currently the case for Xpra and Jupyter.

When clicking the "Share" button, you have to enter a username and confirm. The specific user shall see the session in their Active Sessions overview. The actions are the same, but sharing and file browser should only be possible for session owners. The user can then connect to the session as usual.

During sharing, a dialog with a warning should appear, which raises awareness of risks:

• The owner is fully responsive for the session itself.
• Once shared, it's not possible to "unshare" a session.
• The only way to revoke access is to terminate the session.
• The user, which whom the session is shared, can take full control over the session and behave on behalf of the owner.
• While the session is active, the owner has to track the activity in the session. The session should never be open unattended.