swarm: improve vote system and supports unban

This patch fixes several behaviours:

+ Banning a member now stores the previous state of the member
+ This allow to store empty .crt files for invited
+ Unbanning a member is now possible for an admin
+ This re-add the member in its previous state

https://git.jami.net/savoirfairelinux/jami-product-backlog/-/issues/51
Change-Id: I34d5913c023043e07544f1b8bb6211aea5db0065