Add early feedback to Leader of connection that new keyholders is connecting

[chmurson]

Context
RTC connection handshake happens through server. Both sides (Callee and Caller) are aware that connection is being set up before it fails or is successful.

What
Leader (Callee)
-> Create box -> show a skeleton of a new upcoming key holder connection (for leader and other key holders)
Follower (Caller)
-> Create box -> show early feedback of some kind that connection to earlier is being set up

Leader/Follower (Callee/Caller)
-> Unlock box -> signaling several to be extended to send also information which keyholder is getting connected and display appropriate information in the lobby that the connection is in progress

To think about - Extra
Even during the creation box process, we could extend Signaling Server to pass, not only the fact that the connection is being set up (which already happens), but also some basic information about keyholder (e.g. name) so it can be display instead of simple skeleton.

[tomusdrw]

When the timer starts we probably want to avoid accepting any new connections so that it's not disrupted.

[chmurson]

When the timer starts we probably want to avoid accepting any new connections so that it's not disrupted.

#126 <-- added it here