Automate creation of zulip streams

[MarcoIeni]

#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:

sync-team/src/zulip/mod.rs

Line 152 in dffaba4

.create_user_group(&self.name, &self.description, &self.member_ids)

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.