Remove LiipThemeBundle

As baggy theme was removed and material is the only remaining theme, we don't need a theme switched anymore.
So:
- move all `*.twig` files from the material theme folder to the root
- remove useless translations

src/Wallabag/CoreBundle/DependencyInjection/Configuration.php

@@ -20,9 +20,6 @@ class Configuration implements ConfigurationInterface
                 ->integerNode('items_on_page')
                     ->defaultValue(12)
                 ->end()
-                ->scalarNode('theme')
-                    ->defaultValue('material')
-                ->end()
                 ->scalarNode('language')
                     ->defaultValue('en')
                 ->end()