Create a Tombstone for every deleted resource

[srosset81]

In ActivityPub, it is recommended to replace deleted resources with a Tombstone like this:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Tombstone",
  "formerType": "Event",
  "deleted": "2016-05-03T00:00:00Z",
  "summary": "This resource has been deleted"
}

In ActivityPods, this will make it easier for external apps to know what type of resource has been deleted.

When fetching a Tombstone, we should return the data with a 410 Gone status (to be done on LdpApiService ?)

I suggest to make this optional, so that SemApps instances which don't need this information are not clutered with tombstones (which would make the instance look more like a graveyard !).