Remove ability to change username

Since password are linked to username it was hard to change username while checking that the password is the good one.

Instead of doing crazy things to achieve that, I act that user won't be able to update username.

Also, username (and email) must be unique, since people logged in using username and can request a new password using email.

This should fix #1149

src/Wallabag/CoreBundle/Resources/views/Config/index.html.twig

@@ -84,14 +84,6 @@
     <form action="{{ path('config') }}" method="post" {{ form_enctype(form.user) }}>
         {{ form_errors(form.user) }}
 
-        <fieldset class="w500p inline">
-            <div class="row">
-                {{ form_label(form.user.username) }}
-                {{ form_errors(form.user.username) }}
-                {{ form_widget(form.user.username) }}
-            </div>
-        </fieldset>
-
         <fieldset class="w500p inline">
             <div class="row">
                 {{ form_label(form.user.name) }}