Edit some channel settings

[gnprice]

In the zulip-mobile legacy app, when you're looking at the details of a stream/channel and you're an admin, there's an "Edit stream" button. That takes you to a screen where you can edit the channel's

• name;
• description;
• and channel permissions, i.e. private-with-protected-history vs. private-with-shared-history vs. public vs. web-public.

We should add the same feature here.

The Zulip web app has various other channel settings you can edit; those are out of scope for this issue.

Implementing this properly will require #814, because with new servers "if you're an admin" is no longer the right check for (whether you have permission to edit these settings, and therefore) whether we should show this UI — instead there's a per-channel group-setting value can_administer_channel_group.

Related issues

• Prerequisite for a non-hacky implementation: Implement group-based permission checks #814
• #api design thread on can_administer_channel_group: https://chat.zulip.org/#narrow/channel/378-api-design/topic/can_administer_channel_group/near/1989572