Explicitly record Tombstone quotes as deleted

This adds a `deleted` state to the internal representation, but this does
not change the API, which already included such a state.

spec/lib/activitypub/activity/create_spec.rb

@@ -956,7 +956,7 @@ RSpec.describe ActivityPub::Activity::Create do
           expect(status).to_not be_nil
           expect(status.quote).to_not be_nil
           expect(status.quote).to have_attributes(
-            state: 'pending',
+            state: 'deleted',
             approval_uri: nil
           )
         end