Add a nodeinfo endpoint (#12002)

* Add nodeinfo endpoint

* dont commit stuff from my local dev

* consistant naming since we implimented 2.1 schema

* Add some additional node info stuff

* Add nodeinfo endpoint

* dont commit stuff from my local dev

* consistant naming since we implimented 2.1 schema

* expanding this to include federation info

* codeclimate feedback

* CC feedback

* using activeserializers seems like a good idea...

* get rid of draft 2.1 version

* Reimplement 2.1, also fix metaData -> metadata

* Fix metaData -> metadata here too

* Fix nodeinfo 2.1 tests

* Implement cache for monthly user aggregate

* Useless

* Remove ostatus from the list of supported protocols

* Fix nodeinfo's open_registration reading obsolete setting variable

* Only serialize domain blocks with user-facing limitations

* Do not needlessly list noop severity in nodeinfo

* Only serialize domain blocks info in nodeinfo when they are set to be displayed to everyone

* Enable caching for nodeinfo endpoints

* Fix rendering nodeinfo

* CodeClimate fixes

* Please CodeClimate

* Change InstancePresenter#active_user_count_months for clarity

* Refactor NodeInfoSerializer#metadata

* Remove nodeinfo 2.1 support as the schema doesn't exist

* Clean-up

app/serializers/nodeinfo/discovery_serializer.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class NodeInfo::DiscoverySerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attribute :links
+
+  def links
+    [{ rel: 'http://nodeinfo.diaspora.software/ns/schema/2.0', href: nodeinfo_schema_url }]
+  end
+end

app/serializers/nodeinfo/serializer.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+class NodeInfo::Serializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :version, :software, :protocols, :usage
+
+  def version
+    '2.0'
+  end
+
+  def software
+    { name: 'mastodon', version: Mastodon::Version.to_s }
+  end
+
+  def services
+    { outbound: [], inbound: [] }
+  end
+
+  def protocols
+    %w(activitypub)
+  end
+
+  def usage
+    {
+      users: {
+        total: instance_presenter.user_count,
+        active_month: instance_presenter.active_user_count(4),
+        active_halfyear: instance_presenter.active_user_count(24),
+      },
+
+      local_posts: instance_presenter.status_count,
+    }
+  end
+
+  private
+
+  def instance_presenter
+    @instance_presenter ||= InstancePresenter.new
+  end
+end