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

app/helpers/jsonld_helper.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+module JsonLdHelper
+  def equals_or_includes?(haystack, needle)
+    haystack.is_a?(Array) ? haystack.include?(needle) : haystack == needle
+  end
+
+  def first_of_value(value)
+    value.is_a?(Array) ? value.first : value
+  end
+
+  def supported_context?(json)
+    equals_or_includes?(json['@context'], ActivityPub::TagManager::CONTEXT)
+  end
+
+  def fetch_resource(uri)
+    response = build_request(uri).perform
+    return if response.code != 200
+    Oj.load(response.to_s, mode: :strict)
+  rescue Oj::ParseError
+    nil
+  end
+
+  private
+
+  def build_request(uri)
+    request = Request.new(:get, uri)
+    request.add_headers('Accept' => 'application/activity+json')
+    request
+  end
+end