Only subscribe to likes and comments for boosts

[gnunicorn]

as outlined in #3053 we are annoying users with push notifications on every view -- for which we don't have any decent notifications to show.

That's due to use subscribing to the entire object and its notifications rather than comments and likes specifically. So, what we should be doing instead is:

1. if the user has auto-subscribe on, we subscribe comments and likes of a story or boost when they create them (rather than the generic all notifications)
2. we need to update the notification UI for boosts and comments to a) allow to still unsubscribe from object notifications (if currently activated), but then also have the specific "like notifications" in the configuration bar for any user.

The 2nd part is under the ... on the story / update

and with object notifications activated: