fixes for reasign list

* use existing method in UserRepository and use a method instead of
building a query outside of the repository;
* use renderString to render users
* fix building of filter form (add $data). This make the use of the
method "get" instead of post

src/Bundle/ChillPersonBundle/Resources/views/AccompanyingPeriod/reassign_list.html.twig

@@ -42,8 +42,8 @@
         <h1>{{ block('title') }}</h1>
 
         {{ form_start(form) }}
-        <div class="row filter-box" style="padding: 2rem 2rem 2rem 0;">
-            <div class="col-md-6" style="display: flex; justify-content: space-between;">
+        <div class="row filter-box">
+            <div class="col-md-6">
                 {{ form_label(form.user ) }}
                 {{ form_widget(form.user, {'attr': {'class': 'select2'}}) }}
             </div>