Set default channels per notification type and/or an "enabled by default" flag per channel

[kevinrenskers]

Right now a user gets notification types sent to all registered channels, unless the user specifically disables a channel for a notification type. It should probably be possible to set default channels per notification type and/or an "enabled by default" flag per channel. For example, it might not make sense that all notifications by default are sent by email. Maybe you only want a specific notification type to go email by default, the rest not, while still making it possible for the user to change these defaults.

[kevinrenskers]

The problem is that if we make a channel disabled by default (globally or per notification type), that the user has no way to enable this. Because we only store opt-out in the DisabledNotificationTypeChannel model.