[mod] add a __common__ template that can't be selected but that provides a common place for shared templates.

What has been moved into this template :
* opensearch*.xml is always the same whatever the themes.
* the text inside */about.html

searx/utils.py

@@ -175,6 +175,8 @@ def get_themes(root):
     templates_path = os.path.join(root, 'templates')
 
     themes = os.listdir(os.path.join(static_path, 'themes'))
+    if '__common__' in themes:
+        themes.remove('__common__')
     return static_path, templates_path, themes