flask backend: Add a stub for bitcoin certificates

Returns an empty array. No plans to implement.

backend/flask/src/clipperz/api.py

@@ -659,6 +659,15 @@ class message(HandlerMixin):
         result['result'] = 'done'
         return jsonify({'result': result})
 
+    @login_required
+    def getCertificatesStatus(self, parameters, request):
+        """
+        Provides support for BTC Certificate feature.
+
+        No idea how it works.
+        """
+        return jsonify({'result': {}})
+
 
 class logout(HandlerMixin):