added "create user" functionality (#37)

* added "create user" functionality #5
* improved user admin views
* added Role constraint and validator

app/Resources/views/admin/user_edit.html.twig

@@ -0,0 +1,12 @@
+{% extends 'base.html.twig' %}
+
+{% block page_title %}{{ 'admin_user.title'|trans }}{% endblock %}
+{% block page_subtitle %}{{ 'admin_user.subtitle'|trans }}{% endblock %}
+
+{% block main %}
+    {{ include('default/_form.html.twig', {
+        'title': 'create'|trans,
+        'form': form,
+        'back': path('admin_user')
+    }) }}
+{% endblock %}