Add a dropdown on participate menu, disallow listing participations

Now requires quering the person endpoint to know if an actor
participates in an event, organizers can make authenticated requests to
event { participants { } } to see the pending / approved participants.

Also closes #174

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

js/src/views/Account/Register.vue

@@ -92,6 +92,7 @@ export default class Register extends Vue {
     domain: null,
     feedTokens: [],
     goingToEvents: [],
+    participations: [],
   };
   errors: object = {};
   validationSent: boolean = false;