API user creation behing a toggle

I've added a toggle feature (in internal settings) so that user api creation can be disabled while form registration still can be enabled.
Also, the /api/user endpoint shouldn't require authentication. Even if we check the authentication when sending a GET request, to retrieve current user information.

I've moved all the internal settings definition to config to avoid duplicated place to define them.
I don't know why we didn't did that earlier.

src/Wallabag/CoreBundle/DependencyInjection/Configuration.php

@@ -52,6 +52,17 @@ class Configuration implements ConfigurationInterface
                 ->scalarNode('api_limit_mass_actions')
                     ->defaultValue(10)
                 ->end()
+                ->arrayNode('default_internal_settings')
+                    ->prototype('array')
+                        ->children()
+                            ->scalarNode('name')->end()
+                            ->scalarNode('value')->end()
+                            ->enumNode('section')
+                                ->values(['entry', 'misc', 'api', 'analytics', 'export', 'import'])
+                            ->end()
+                        ->end()
+                    ->end()
+                ->end()
             ->end()
         ;