Add update config per channel

[davidanthoff]

At the moment we can't configure the update behavior of individual channels. Instead, whenever a channel is outdated, it will show a reminder to update the channel.

Instead, we might want to add configuration per channel, a la auto (automatically update the channel), prompt (prompt for updating a channel when there is a new version), remind (just remind that there is an update, without UI to update, that is what we have at the moment) and never (just don't bug a user with any update info).