read max length from config

prolefeeder.py

@@ -18,7 +18,7 @@ Bootstrap(app)
 class DownloadForm(FlaskForm):
     start_time = DateTimeField('Start time', validators=[DataRequired()])
     length = DecimalField('Length', validators=[DataRequired(),
-                                                NumberRange(min=1, max=180)])
+                                                NumberRange(min=1, max=app.config['MAX_LENGTH'])])
     submit = SubmitField('Download')