all-channels: Compute channel colors client-side, for unsubscribed channels #1848
Description
The v1 of an "All channels" page (#188) will use gray for any channels the self-user is not subscribed to, because the server doesn't assign a color to unsubscribed channels.
The web app seems to choose colors client-side; we should do the same. See screenshot from web, showing some subscribed and some unsubscribed channels:
