Request to join group

This pull request addresses #458
Not sure if it is simpler though. You cannot send to all clients the list of all pending members. So you would have to play with making the list only display the entry for the user themself. See how we are sending name of imported file in personal library to the user.

So having a list of all pending groups might be useful. But sending the whole list to the client for each client sound a bit unnecessary. But we can optimize those things later as well.

So you decide.