Define missing JSON-LD properties (#4767)

Using _: property names is discouraged, as in the future,
canonicalization may throw an error when encountering that instead
of discarding it silently like it does now.

We are defining some ActivityStreams properties which we expect
to land in ActivityStreams eventually, to ensure that future versions
of Mastodon will remain compatible with this even once that happens.
Those would be `locked`, `sensitive` and `Hashtag`

We are defining a custom context inline for some properties which we
do not expect to land in any other context. `atomUri`, `inReplyToAtomUri`
and `conversation` are part of the custom defined OStatus context.

app/serializers/activitypub/delete_serializer.rb

@@ -2,8 +2,7 @@
 
 class ActivityPub::DeleteSerializer < ActiveModel::Serializer
   class TombstoneSerializer < ActiveModel::Serializer
-    attributes :id, :type
-    attribute :atom_uri, key: '_:atomUri'
+    attributes :id, :type, :atom_uri
 
     def id
       ActivityPub::TagManager.instance.uri_for(object)