conversation: don't connect to banned members

This patch makes two changes to ensure that banned swarm members don't
sync with or receive message notifications from other members:
1) when bootstrapping a conversation, devices belonging to banned
   members are no longer added to the swarm manager;
2) when merging a "ban" commit for a certain swarm member, that member
   is immediately removed from the swarm manager's routing table and all
   Git sockets with them are removed.

GitLab: #1041
Change-Id: Idb817795f978ca4ca46502efea5101831ffccbe3