Automate creation of zulip streams

[marcoieni]

rust-lang/sync-team#92 will automate member addition / removal from a stream.
But it assumes that the stream already exists.

Edit sync-team to automate the stream creation if it doesn't exist already. You can take inspiration from here:

https://github.com/rust-lang/sync-team/blob/dffaba4155fac3a83a08cfbe21dfd05880202d5c/src/zulip/mod.rs#L152

You should also do a PR in the team repo to update the schema.
Keep in mind that zulip streams can be private or public. Maybe we need a boolean to distinguish between the two.

Question: should we only automate the creation of private streams?