Offer subscribe/unsubscribe in channel action sheet

[chrisbobbe]

As with #347 and #1223, I think a "channel action sheet" in the style of the existing "topic action sheet" would be a fine place for this.

The button can say "Subscribe" when not subscribed, and "Unsubscribe" when subscribed.

The "Unsubscribe" button should probably give a confirmation modal before making the API request.

Related issues

• UI for channel muting/unmuting #347
• UI for channel subscription settings other than muted/unmuted #1223

[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.