Add update config per channel

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 Instead, whenever a channel is outdated, it will show a reminder to update the channel.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).

No files, tests, or entry points are identified. Start by locating the channel update and reminder handling and the existing configuration path; confirm the four requested behaviors and their completion criteria with maintainers before making changes.

Written by the indexing model from the issue text.