Viewers federation protocol v2

More efficient than the current one where instance is not fast enough to
send all viewers if a video becomes popular

The new protocol can be enabled by setting env
USE_VIEWERS_FEDERATION_V2='true'

Introduce a result field in View activity that contains the number of
viewers. This field is used by the origin instance to send the total
viewers on the video to remote instances. The difference with the
current protocol is that we don't have to send viewers individually to
remote instances.

There are 4 cases:
 * View activity from federation on Remote Video -> instance replaces
   all current viewers by a new viewer that contains the result counter
 * View activity from federation on Local Video -> instance adds the
   viewer without considering the result counter
 * Local view on Remote Video -> instance adds the viewer and send it to
   the origin instance
 * Local view on Local Video -> instance adds the viewer

Periodically PeerTube cleanups expired viewers. On local videos, the
instance sends to remote instances a View activity with the result
counter so they can update their viewers counter for that particular
video

server/core/helpers/activity-pub-utils.ts

@@ -196,11 +196,17 @@ const contextStore: { [ id in ContextType ]: (string | { [ id: string ]: string
     uuid: 'sc:identifier'
   }),
 
+  View: buildContext({
+    WatchAction: 'sc:WatchAction',
+    InteractionCounter: 'sc:InteractionCounter',
+    interactionType: 'sc:interactionType',
+    userInteractionCount: 'sc:userInteractionCount'
+  }),
+
   Collection: buildContext(),
   Follow: buildContext(),
   Reject: buildContext(),
   Accept: buildContext(),
-  View: buildContext(),
   Announce: buildContext(),
   Comment: buildContext(),
   Delete: buildContext(),