Force themes to use .twig as file extension

We recommend plugin developers to use templates when serving HTML contents (like the UI of PicoAdmin), however, by supporting multiple file extensions for themes, we make it pretty hard to overwrite a plugin's template with a theme. As always, we preserve BC using PicoDeprecated.

lib/Pico.php

@@ -1991,14 +1991,7 @@ class Pico
     protected function getTwigTemplate()
     {
         $templateName = !empty($this->meta['template']) ? $this->meta['template'] : 'index';
-
-        if (file_exists($this->getThemesDir() . $this->getConfig('theme') . '/' . $templateName . '.twig')) {
-            $templateName .= '.twig';
-        } else {
-            $templateName .= '.html';
-        }
-
-        return $templateName;
+        return $templateName . '.twig';
     }
 
     /**