Add ActivityPub inbox (#4216)

* Add ActivityPub inbox

* Handle ActivityPub deletes

* Handle ActivityPub creates

* Handle ActivityPub announces

* Stubs for handling all activities that need to be handled

* Add ActivityPub actor resolving

* Handle conversation URI passing in ActivityPub

* Handle content language in ActivityPub

* Send accept header when fetching actor, handle JSON parse errors

* Test for ActivityPub::FetchRemoteAccountService

* Handle public key and icon/image when embedded/as array/as resolvable URI

* Implement ActivityPub::FetchRemoteStatusService

* Add stubs for more interactions

* Undo activities implemented

* Handle out of order activities

* Hook up ActivityPub to ResolveRemoteAccountService, handle
Update Account activities

* Add fragment IDs to all transient activity serializers

* Add tests and fixes

* Add stubs for missing tests

* Add more tests

* Add more tests

spec/helpers/jsonld_helper_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe JsonLdHelper do
+  describe '#equals_or_includes?' do
+    it 'returns true when value equals' do
+      expect(helper.equals_or_includes?('foo', 'foo')).to be true
+    end
+
+    it 'returns false when value does not equal' do
+      expect(helper.equals_or_includes?('foo', 'bar')).to be false
+    end
+
+    it 'returns true when value is included' do
+      expect(helper.equals_or_includes?(%w(foo baz), 'foo')).to be true
+    end
+
+    it 'returns false when value is not included' do
+      expect(helper.equals_or_includes?(%w(foo baz), 'bar')).to be false
+    end
+  end
+
+  describe '#first_of_value' do
+    pending
+  end
+
+  describe '#supported_context?' do
+    pending
+  end
+
+  describe '#fetch_resource' do
+    pending
+  end
+end