Notifications: Add Feedback settings

In order to avoid doing something in the Shell that we'll change later we need to make the following changes in Settings:

• Change the "Notification Popups" setting to "Do Not Disturb", and flip the switch values (on = no notifications, off by default). When Do Not Disturb is on, the feedbackd profile should be set to "silent".
• Add a "Notification Feedback" settings row with a dropdown setting the preferred feedbackd profile. This row would be grayed out when Do Not Disturb is on (since that temporarily overrides the preferred profile).

If it's easy enough to do I'd also

• Remove the dialog on the individual apps and have a single switch that turns notifications on or off
• Remove the "Lockscreen Notifications" toggle at the top

cc @guido.gunther @sam.hewitt @mohammed.sadiq @julian.sparber