Add showing and modifying user name (#306)

Make sure to use the user name field everywhere

Add showing and modifying user name

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/frontend/pulls/306
Co-Authored-By: konrad <konrad@kola-entertainments.de>
Co-Committed-By: konrad <konrad@kola-entertainments.de>

src/components/tasks/edit-task.vue

@@ -107,7 +107,7 @@
 			<label class="label" for="">Assignees</label>
 			<ul class="assingees">
 				<li :key="a.id" v-for="(a, index) in taskEditTask.assignees">
-					{{ a.username }}
+					{{ a.getDisplayName() }}
 					<a @click="deleteAssigneeByIndex(index)">
 						<icon icon="times"/>
 					</a>