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/components/Event/DateTimePicker.vue

@@ -6,7 +6,7 @@
 </template>
 <script lang="ts">
 import { Component, Prop, Vue, Watch } from 'vue-property-decorator';
-@Component({})
+@Component
 export default class DateTimePicker extends Vue {
   @Prop({ required: true, type: Date }) value!: Date;
   @Prop({ required: false, type: String, default: 'Datetime' }) label!: string;