Support new commentsEnabled property

This is a field previously supported by PeerTube, and just recently
added on PixelFed, that should inform other ActivityPub services whether
or not comments are enabled on any given post. WriteFreely doesn't
support comments today, so this will always be false.

posts.go

@@ -1021,7 +1021,10 @@ func fetchPost(app *app, w http.ResponseWriter, r *http.Request) error {
 
 		p.Collection = &CollectionObj{Collection: *coll}
 		po := p.ActivityObject()
-		po.Context = []interface{}{activitystreams.Namespace}
+		po.Context = []interface{}{
+			activitystreams.Namespace,
+			activitystreams.Extensions,
+		}
 		return impart.RenderActivityJSON(w, po, http.StatusOK)
 	}
 
@@ -1091,6 +1094,9 @@ func (p *PublicPost) ActivityObject() *activitystreams.Object {
 			})
 		}
 	}
+
+	o.CommentsEnabled = false
+
 	return o
 }
 
@@ -1300,7 +1306,10 @@ func viewCollectionPost(app *app, w http.ResponseWriter, r *http.Request) error
 	} else if strings.Contains(r.Header.Get("Accept"), "application/activity+json") {
 		p.extractData()
 		ap := p.ActivityObject()
-		ap.Context = []interface{}{activitystreams.Namespace}
+		ap.Context = []interface{}{
+			activitystreams.Namespace,
+			activitystreams.Extensions,
+		}
 		return impart.RenderActivityJSON(w, ap, http.StatusOK)
 	} else {
 		p.extractData()