Make team chat channels opt out instead of opt in

Currently when you add a new team member, they only get subscribed to #general by default, and will not even be aware of the existence of additional channels unless they click the gear next to the team name in the chat interface. IMHO, it would be a lot better if new team members were automatically subscribed to all the channels in the team upon joining. Existing team members should also automatically be added to new chat channels that are created.

I think at the very least both of these things should be options that can be enabled for a team.

Start by locating the team-member onboarding and chat-channel creation entry points in the Keybase client. Clarify whether automatic subscriptions should be enabled by default or controlled by separate team options, and define behavior for existing members and newly created channels. Done means the chosen subscription behavior works consistently for both cases.

Written by the indexing model from the issue text.