Support creating a new channel

[chrisbobbe]

It's possible to create a channel in the legacy app, and we've had a request for this feature in beta feedback: #mobile > things where I couldn't tell if it was a feature or a bug @ 💬

[PIG208]

This will be a reasonable follow-up to #188. There is a "New channel" button in the Figma design, but no page has been created yet.

[chrisbobbe]

When this is done we should update the Help Center doc. We removed instructions for the legacy app when preparing to launch the new app: zulip/zulip@1192491