Optionally hide previews in notifications

[danirabbit]

This is a privacy feature offered on mobile and I feel like it's worth considering.

This setting should ignore images and body text sent to the notification server. Images because they can be private information like avatars. Body text should be optional and often contains specific information, message previews, etc.

We should be careful to make sure that the notifications our apps send don't expose sensitive information in their titles.

Critical notifications should ignore this setting

[cassidyjames]

@danrabbit another way I've seen it implemented is that it'll just show the app icon and app name instead of the original contents at all—this way app developers don't have to worry about the body being treated as sensitive while the title is not.