Support creating a new channel #1572

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 @ 💬

We should check the appropriate permissions before offering the button.

Implementation

• For the modern permissions, search in https://zulip.com/api/register-queue for:
  • realm_can_create_public_channel_group
  • realm_can_create_private_channel_group
  • realm_can_create_web_public_channel_group
• For pre-server 9 legacy permissions, search in https://zulip.com/api/register-queue for:
  • realm_create_public_stream_policy
  • realm_create_private_stream_policy
  • realm_create_web_public_stream_policy