Fix number of participant inconsistency

[JulienVig]

Fixes #894 identified by @christinakopi.

We used to update the number of participants only during weight sharing. I made a few changes to update the participants more frequently (doesn't cover every case yet):

• Client emits a participants event (which is forwarded by Disco) that the webapp can watch
• The server now sends the number of participants at connection, in WaitForMoreParticipants messages and in EnoughParticipants messages
• With tests©
• The number of participants is not dynamically updated when clients are waiting for more participants, it stays fixed until enough participants joined and the training can start (we can implement a new message type to update clients when a new client joins).

[tharvik]

haa, works flawlessly, niceux!