Change User class string representation to username

Having the username in the API is more useful than the display name.
We keep the display name in the UI.

Fix INFRA-241

app/models.py

@@ -138,7 +138,7 @@ class User(db.Model, UserMixin):
         return bool(set(self.groups) & set(names))
 
     def __str__(self):
-        return self.display_name
+        return self.username
 
     def to_dict(self):
         return {

app/templates/base.html

@@ -35,7 +35,7 @@
           {% if current_user.is_authenticated %}
           <div class="dropdown {{ is_active(path == "/user/profile") }}">
             <a class="nav-link dropdown-toggle" href="#" id="navbarDropdownMenuLink" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
-              {{current_user}}
+              {{ current_user.display_name }}
             </a>
             <div class="dropdown-menu" aria-labelledby="navbarDropdownMenuLink">
               <a class="dropdown-item" href="{{ url_for('user.profile') }}">Profile</a>