Add create note subscription api endpoint

app/controllers/api/note_subscriptions_controller.rb

@@ -0,0 +1,18 @@
+module Api
+  class NoteSubscriptionsController < ApiController
+    before_action :check_api_writable
+    before_action :authorize
+
+    authorize_resource
+
+    def create
+      note_id = params[:note_id].to_i
+      note = Note.find(note_id)
+      note.subscribers << current_user
+    rescue ActiveRecord::RecordNotFound
+      report_error "Note #{note_id} not found.", :not_found
+    rescue ActiveRecord::RecordNotUnique
+      report_error "You are already subscribed to note #{note_id}.", :conflict
+    end
+  end
+end