swarm: avoid duplicated conversation after a ban

Several improvements are included with this patch:

1. confirmed is resetted when account is removed, this allow to
send a request when conversation is added back and reset the
whole matrix, avoiding duplicates
2. Banned can be due to harrasement. In this case, keeping the
conversation like the old behavior (pre-swarm) is better, to keep
proofs. Also re-adding back a contact is immediate if it was a
mistake as the conversation is kept back.

jami-project#1449

Change-Id: I13da8ce9bd431b91ce7b7d455dae561358c62f10