Mart activity-streams-specs

Currently the spec says nothing about privacy of activities.

While the problem is partially solved by simply not exposing activities to those who can't see them, it would be useful to include some kind of indication that the activity is non-public for those who can see it.

As an example, when an entry is posted as "Friends Only" on LiveJournal, those who have access to see it will see it with an icon indicating that it is friends only, which lets the reader know that this is sensitive information which should perhaps not be discussed outside of the context of this entry.

Unfortunately currently LiveJournal does not manifest this in feeds, and some sites will have a slightly different concept to "friends only". We could consider a simple "is non-public" flag to begin with, though.