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/Entity/Config.php

@@ -35,14 +35,6 @@ class Config
      */
     private $id;
 
-    /**
-     * @var string
-     *
-     * @Assert\NotBlank()
-     * @ORM\Column(name="theme", type="string", nullable=false)
-     */
-    private $theme;
-
     /**
      * @var int
      *
@@ -162,30 +154,6 @@ class Config
         return $this->id;
     }
 
-    /**
-     * Set theme.
-     *
-     * @param string $theme
-     *
-     * @return Config
-     */
-    public function setTheme($theme)
-    {
-        $this->theme = $theme;
-
-        return $this;
-    }
-
-    /**
-     * Get theme.
-     *
-     * @return string
-     */
-    public function getTheme()
-    {
-        return $this->theme;
-    }
-
     /**
      * Set itemsPerPage.
      *