message: allow non-relative LOGO_TARGET_PATH
diff: {% set WARNING_MSG = appbuilder.app.config.get('WARNING_MSG') %}
  {% set app_icon_width = appbuilder.app.config.get('APP_ICON_WIDTH', 126) %}
  {% set logo_target_path = appbuilder.app.config.get('LOGO_TARGET_PATH') or '/profile/{}/'.format(current_user.username) %}
  +{% set root_path = logo_target_path if not logo_target_path.startswith('/') else '/superset' + logo_target_path if current_user.username is defined else '#' %}
  <div class="navbar navbar-static-top {{menu.extra_classes}}" role="navigation">
  <div class="container-fluid">
  <span class="icon-bar"></span>
  <span class="icon-bar"></span>
  </button>
  - <a class="navbar-brand" href="{{ '/superset' + logo_target_path if current_user.username is defined else '#' }}">
  + <a class="navbar-brand" href="{{ root_path }}">
  <img
  width="{{ app_icon_width }}"
  src="{{ appbuilder.app_icon }}"
  