Allow users to change their display name

Fixes #269

app/templates/users/delete.html

@@ -1,7 +1,7 @@
 {% extends "base.html" %}
 
 {% block title %}
-	Delete user {{ user.display_name }}
+	Delete user {{ user.username }}
 {% endblock %}
 
 {% block content %}

app/templates/users/list.html

@@ -49,6 +49,11 @@
 
 				<span class="col-sm {{ user.rank }}">
 					{{ user.display_name }}
+					{% if user.username != user.display_name %}
+						<span class="text-muted small ml-2">
+							({{ user.username }})
+						</span>
+					{% endif %}
 				</span>
 
 				<div class="col-sm-1 text-center">

app/templates/users/profile_edit.html

@@ -48,6 +48,10 @@
 <form action="" method="POST" class="form" role="form">
 	{{ form.hidden_tag() }}
 
+	{% if user.checkPerm(current_user, "CHANGE_DISPLAY_NAME") %}
+		{{ render_field(form.display_name, tabindex=230, hint=_("Pretending to be another user is grounds for a permanent ban")) }}
+	{% endif %}
+
 	{% if user.checkPerm(current_user, "CHANGE_PROFILE_URLS") %}
 		{{ render_field(form.website_url, tabindex=232) }}
 		{{ render_field(form.donate_url, tabindex=233) }}