Offer "Copy link to channel" in channel action sheet

[chrisbobbe]

We haven't yet built a channel action sheet, but I think we plan to, as a home for

• UI for channel muting/unmuting #347
• UI for channel subscription settings other than muted/unmuted #1223
• Offer subscribe/unsubscribe in channel action sheet #1224
• Offer "Mark channel as read" in channel action sheet #1226

The action sheet should offer a "Copy link to channel" button, like it does in the legacy apps.

Related issues

• Offer "Copy link to topic" #792

[chrisbobbe]

This came up in beta feedback on CZO:

In the channel list, a long press on the channel name should bring up a menu with options like "Mark all read" and "unsubscribe" and "copy link". That menu was available in the old app.

[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@3a62603